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Soil Water
Dean E. Eisenhauer, Derrel L. Martin, Derek M. Heeren, Glenn J. Hoffman
Pages 11-33 (doi: 10.13031/ISM.2021.2) in Irrigation Systems Management. ,
Abstract. See https://www.asabe.org/ISM for a PDF file of this entire textbook at no cost.
2.1 Introduction
Like humans, plants need water to survive. But, from where do plants get their water? Well, the soil beneath your feet is the answer. The soil within the plant root zone serves as a reservoir for storing precipitation and irrigation water for future use by plants. Effective management of an irrigation system requires the understanding and use of the basic concepts of soil water. Without an adequate understanding of these concepts, the irrigator will not know how much water to apply or when to irrigate. These are fundamental concerns in irrigation management. After a review of these concepts, water entry into the soil will be discussed followed by a presentation of field methods for measuring soil water.
The goal of irrigation management is to maintain the amount of water in the soil between wet and dry limits to satisfy the plants water requirements. The wet soil limit occurs when plants suffer because of decreased aeration, and the dry soil limit occurs when plants have difficulty obtaining the water they need. Thus, it is necessary to determine the amount of soil water available to plants and the proper amount of irrigation water to be applied.
Two measures of soil water are important for managing irrigation systems. The first is the amount of water in the soil, which is commonly referred to as the soil water content. The second property is the soil water potential, which is a measure of how hard plants have to work to remove water from the soil.
Before considering these two measures of soil water, the impact of the components of the soil and their impact on the ability of soil to store water must be understood. The basic components of the soil are the solid mineral particles, organic matter, the voids among the particles, and water and air occupying the voids. The capacity of the soil to store water depends upon the volume of the voids present.
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| Figure 2.1. Composition of a soil volume. |
2.2 Soil Composition
As Figure 2.1 illustrates, soil is composed of three major components: soil particles, air, and water. The fractions of water and air are contained in the voids between soil particles. The ratio of the volume of pores (voids) to the total (bulk) volume of a soil is the porosity (f). One way to determine porosity is to measure the volume of a soil that is composed of soil particles and the fraction made up of the pores. The porosity may also be determined using the soil bulk density (?b) Bulk density is the density of the undisturbed (bulk) soil sample described by:
(2.1)
where: ?b = soil bulk density (g/cm3),
Ms = mass of dry soil (g), and
Vb = volume of bulk soil sample (cm3).
Given the bulk density of a soil, the porosity (in percent) can be calculated as:
(2.2)
where: f = soil porosity
?p = soil particle density (a common value for mineral soils is 2.65 g/cm3).
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| Figure 2.2. USDA soil textural triangle. |
The mineral fraction of the soil volume is composed of sand, silt, and clay separates. With the USDA classification system, the equivalent diameter size limits are: Clay < 0.002 mm, silt 0.002 to 0.05 mm, and sands 0.05 to 2.0 mm. The relative proportions of the various soil separates are used to define the soil texture using the USDA soil textural triangle shown in Figure 2.2. These textural classes are referred to frequently in this book.
2.3 Soil Water Content
The amount of water in a soil can be expressed in many ways, including a dry soil basis (mass water content), a volumetric basis (volumetric water content), fraction of the available water remaining, and fraction of the available water depleted. With so many different terms, confusion is bound to arise. Irrigation managers must understand all of these terms to interpret soil water status correctly.
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| Figure 2.3. Concept of mass water content. |
The mass water content or gravimetric water content (?m) is the ratio of the mass of water in a sample to the dry soil mass, expressed as either a decimal fraction or as a percentage. Mass water content is determined by weighing a field soil sample, drying the sample for at least 24 hours at 105°C, and then weighing the dry soil. The decrease in mass of the sample due to drying represents the mass of water in the soil sample. The mass of the sample after drying represents the mass of dry soil. The mass water content is found by:
(2.3)
where: ?m = mass water content,
Mw= mass of water lost during drying (g), and
Ms = mass of dry soil (g).
Figure 2.3 illustrates the relationship between the weight of the water in the soil and the dry weight of the soil when determining mass water content.
The volumetric water content (?v) represents the volume of water contained in a volume of undisturbed soil. The volumetric water content is defined as:
(2.4)
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Figure 2.4. Concept of volumetric water content. |
where: ?v = volumetric water content,
Vw = volume of water (cm3), and
Vb = bulk volume of soil sample (cm3).
Figure 2.4 illustrates the volume of the components needed to calculate ?v.
To find the volumetric water content, the volume of the undisturbed soil sample must be determined, which is sometimes difficult to measure. The mass water content is more easily determined but is often not as useful as the volumetric water content. Therefore, the following equation, which connects mass water content to volumetric water content, is convenient.
(2.5)
where: ?w= density of water, which is 1 g/cm3.
When comparing water amounts per unit of land area, it is frequently more convenient to speak in equivalent depths of water rather than water content. The relationship between volumetric water content and the equivalent depth of water in a soil layer is:
d = ?vL (2.6)
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| Figure 2.5. Concept of depth of water contained in a soil layer. |
where: d = equivalent depth of water in a soil layer (cm)
L = depth increment of the soil layer (cm).
Figure 2.5 illustrates the concept of equivalent depth of water per depth of soil. This calculation is very useful in irrigation scheduling which will be discussed in Chapter 6. In Figure 2.5, p is a constant equal to 3.14 and r is the radius of the cylindrical sample.
2.4 Soil Water Potential
The amount of water in the soil is not the only concern in irrigation management. Plants must be able to extract water from the soil. Soil water potential (?t) is an indicator or measure of the energy status of soil water relative to that of water at a standard reference (Hillel, 1980). This energy is due to the position of the water relative to the reference and the internal state of the water and is often expressed as energy per unit of volume (pressure) or energy per unit of weight (head). Common units of pressure and head, and their equivalents are shown in Table 2.1. The standard reference is often denoted at a high energy level and assigned a value of zero. Thus, soil water potential and its components can all have negative values. A high energy level of water potential will have a smaller negative value (lower magnitude) than a water potential at a lower energy level. For example, in a wet soil, matric potential (discussed below), ?m, will have a small negative value, say ?m = 0.3 bar, while in a dry soil ?m may be 15 bars.
| Table 2.1. Common units of pressure and head and their equivalents. | ||
| Unit | Pressure Equivalent | Water Head Equivalent |
| 1 atmosphere | 101.3 kPa (kilopascals) | 1034 cm H2O |
| (atm) | 1.013 bar | 34 ft H2O |
| 101.3 cb (centibar) | 76 cm Hg | |
| 14.7 psi (lb/in2) | 29.9 in Hg | |
| 1 psi (lb/in2) | 6.89 kPa | 2.31 ft H2O |
The three major components of soil water potential are gravitational potential (?g), matric potential (?m), and osmotic potential (?o). The soil water potential (?t) then is:
(2.7)
Equation 2.7 ignores the impact of overburden pressure on soil water potential. The gravitational potential is due to the force of gravity pulling downward on the water in the soil. Matric potential is a result of the forces the soil particles place on the water by adhesion and surface tension at the soil-air interface. These combined forces cause capillarity, which is sometimes referred to as soil water tension. Soil water tension is expressed as a positive value. Osmotic potential is caused by dissolved solids (salts) in the soil water. The osmotic potential affects the availability and movement of water in soils when a semipermeable membrane (like plant roots) is present. This topic is discussed in more detail in Chapter 7.
Where rainfall is significant and irrigation water is nearly free of salts, the concentration of salts in the soil is generally low, so the osmotic potential is near zero. The osmotic potential does not influence the flow of water through the soil profile. It does, however, have an effect on water uptake by plants and on evaporation. During evaporation, water changes from liquid to vapor at the soil-air interface near the soil surface but salts are left behind in the soil. The higher the salt content of soil water (lower osmotic potential) the lower the rate of evaporation. Water uptake through plant roots is also influenced by the osmotic potential; the higher the salt concentration in the soil solution the more work a plant has to do to absorb water from the soil. Thus, where soil salinity is appreciable, osmotic potential must be considered for evaluating plant water uptake or where water vapor flow is important.
The component of soil water potential that dominates the release of water from soil to plants when salts are minimal is the matric potential. Several forces are involved in the retention of water by the soil matrix. The most strongly held water is adsorbed around soil particles by electrical forces. This water is typically held too tightly for plants to extract. Water is also held in the pores between soil particles by a combination of attractive (surface tension) and adhesive forces. The strength of the attractive force depends on the sizes of the soil pores. Large pores will freely give up pore water to plants due to the much higher matric potential in the soil or to drainage due to the gravitational potential (Martin et al., 2017). For a given amount of water in a particular soil, there will be a corresponding matric water potential. Here we will express the magnitude of the matric potential as soil water tension thus in the positive realm. The curve representing the relationship between the water tension within the soil and its volumetric water content is referred to as the soil water release or soil water retention curve. The soil water release curves in Figure 2.6 show that water is released (volumetric water content is reduced) by the soil as the tension increases.
Soil water release curves are often used to define the amount of water available to plants. Two terms are used to define the upper and lower limits of water availability. The upper limit, field capacity (?fc), is defined as the soil water content where the drainage rate, caused by gravity, becomes negligible. Thus, the soil is holding all of the water it can without any significant loss due to drainage. The permanent wilting point (?wp), the lower limit, is the water content below which plants can no longer extract water from the soil. At this point (WP) and at higher tension values, plants will wilt permanently and will not recover if the water stress is relieved. Neither of these two limits are exact. The WP has traditionally been defined as the water content corresponding to 15 bars of soil water tension or 1,500 cb. This is a reasonable working definition because the water content varies only slightly over a wide range of soil water tension near 15 bars. For example, if the plants permanently wilt at 20 bars of tension, the water content is not much different than at 15 bars and the error in the estimate of water available to plants is small. Example values of ?fc and ?wp are given in Figure 2.6 for several soil types.
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| Figure 2.6. Example soil water release curves for three soil textures showing the values of ?s, ?fc, and ?wp for each soil. |
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| Figure 2.7. Graphical representation of free-draining water, water available for plant uptake, and unavailable water on a soil water release curve. |
Field capacity is often considered to be the water content at a matric potential of minus one-third bar or a tension of 33 cb. This is not a good definition for all soils. This tension value for FC is fairly good for some fine-textured soils but is too large a tension for medium- and coarse-textured soils. The field capacity values shown in Figure 2.6 are more representative than a strict one-third bar definition. Methods will be presented later where the actual field conditions will be used to estimate field capacity for irrigation management.
Users of soil water measurements must keep in mind that there is a difference between volumetric water content at field capacity (?fc) and volumetric water content at saturation (?s). If the voids are completely filled with water and air is absent, the soil is said to be saturated. The volumetric water content equals porosity at saturation, i.e., ?s = f. As gravity causes drainage to occur, air enters the soil and soil water content reaches ?fc as drainage from gravity ceases. Thus, ?fc is less than ?s.
The relationships among the soil water that is free to drain due to gravity, the soil water available for plant water use, and the soil water that is not available to be extracted by plant roots is illustrated in Figure 2.7 on a soil water release curve. The water that is free to drain by gravity is between ?s and ?fc. Available water is that water between ?fc and ?wp, and unavailable water is that water between ?wp and 0.
2.5 Available Water and the Soil Water Reservoir
Irrigation managers can view the soil as a reservoir for holding water. Figure 2.8 illustrates the analogy between a reservoir and a soil. Soil without any water would be like an empty reservoir (Figure 2.8a). Pores in the soil, measured as porosity, provide space for the storage of water. When saturated, the entire void space (reservoir) is filled with water as in Figure 2.8b. After 1 to 3 days of drainage, the water content reaches field capacity (Figure 2.8c). Water leaves through the drain tube on the side by gravity in the analogy. A plant can easily extract water between field capacity and a specific water content represented by the bottom of the large tube extending into the reservoir (Figure 2.8d). This specific water content is referred to as minimum balance and the difference between field capacity and minimum balance is allowable depletion (AD). As the water content decreases below the minimum balance (Figure 2.8e), the plant must work harder to extract the water it requires. The stress a plant experiences below the minimum balance causes a reduction in yield potential. If the reservoir is not replenished, the water content will continue to decrease and eventually reach permanent wilting, which is represented by the bottom of the small tube in Figure 2.8f. Once this point is reached, a plant can no longer recover even if water is added.
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| Figure 2.8. Reservoir analogy of soil water. |
The water held between field capacity and the permanent wilting point is called the available water capacity (AWC), i.e., available for plant use. The AWC of the soil is expressed in units of depth of available water per unit depth of soil, for example in/in or cm/cm. The AWC is calculated by:
(2.8)
For the fine sandy loam soil shown in Figure 2.6, the volumetric water content at field capacity (?fc) is 0.23 and the volumetric water content at WP (?wp) is 0.10. Thus, the available water capacity for that soil is 0.13 in/in or cm/cm (0.23 0.10). You should read this as 0.13 inches of water per inch of soil depth.Field soils are generally at a water content between the FC and the WP. Commonly used terminology in irrigation management is soil water depletion (SWD) or soil water deficit (SWD). SWD refers to how much of the available water has been removed, i.e., the difference between ?fc and ?v, the actual soil water content. The difference between ?v and ?wp is the amount of available water remaining.
Often the depleted and remaining water values are expressed as a fraction or percentage. The equations for determining the fraction of available water depleted and the fraction of available water remaining are as follows:
fraction of available water depleted 
(2.9)
and fraction of available water remaining 
(2.10)
Also, 
(2.11a)
or 
(2.11b)
It is very useful in irrigation management to know the depth of water required to fill a layer of soil to field capacity. This depth is equal to the SWD. Do you see why? SWD can be calculated by:
(2.12)
By substituting Equation 2.9 into Equation 2.12 you will find that this is equivalent to:
(2.13)
The capacity of the available soil water reservoir, total available water (TAW), depends on both the AWC and the depth that the plant roots have penetrated. The relationship is:
(2.14)
where: TAW = total available water capacity within the plant root zone (cm), and
Rd = depth of the plant root zone (cm).
Plant root zone depths will be discussed further in Chapter 6. Equation 2.14 is applicable to soils that have the same soil texture throughout the root zone. In the field, soil textures change with soil depth. Thus, TAW is calculated by determining SWD for each soil layer throughout the root zone and adding them together.
2.6 Determining Available Water Capacity
The values of ?fc and ?wp of a soil used to calculate AWC, can be determined by field and laboratory methods. Discussion of the various techniques to measure these variables is beyond the scope of this book. The reader should refer to Bruce and Luxmore (1986) and Klute (1986) and references therein for detailed information. Relatively simple experiments for approximating these variables are explained below.
Field capacity may be determined by flooding a small area of land, covering it to suppress evaporation, waiting several days for drainage to become negligible, and then sampling to determine the water content throughout the soil profile. When flooding ceases, the water content falls rapidly as the largest soil pores are quickly drained by gravity. After the rate of drainage slows in 1 to 3 days, the water content remains nearly constant. This is field capacity. At this time, the soil should be sampled for water content. As a rule of thumb, 1 day of drainage will generally be adequate for sandy soils, 2 days for silt loam soils, and 3 days for silty clay loam soils. A simpler field method of determining field capacity is to take soil samples at intervals following a thorough irrigation or rain in a fallow field. When ?v remains nearly constant the value is ?fc.
The water content at WP can be determined by measurements in areas where the available soil water has been exhausted. In this case, an area that experiences severe water stress would be a good location to take a soil sample. The sample could be analyzed for ?v at that time to determine the ?wp throughout the soil profile.
If field capacity is known, ?wp can be estimated by subtracting AWC from ?fc. Suppose the ?fc is 0.30 in/in and the AWC is 0.18 in/in. Wilting point, ?wp is then 0.30 minus 0.18 or 0.12 in/in. Often, AWC is tabulated in soil survey reports and textbooks.
Generally in irrigation management, the same value of ?wp is used throughout the root zone for calculating water requirements. At the same time, we use root zones shallower than what is explored by plant roots. This creates a margin of safety, and to some extent, accounts for the fact that the permanent wilting point in the upper portion of the root zone is often higher than in the lower portion. This simplification makes water balance calculations much easier and has worked well in scheduling and designing irrigation systems.
2.7 Tabulated Values of Typical Soil Properties
Data for soil properties are available from various sources. For example, in the U.S., county-level Soil Survey Reports normally list many of the soil properties described in this chapter. These reports are available electronically for application with geographic information systems and on the internet such as the Web Soil Survey (http://websoilsurvey.nrcs.usda.gov/app/). An example listing is shown in Table 2.2.
Generalized values of AWC, ?fc, and ?wp for a range of soil textures are given in Table 2.3.
| Table 2.2. Examples of soil properties for Platte County, Nebraska (USDA, 1988). | |||||
| Soil Name | Soil Depth(in) | USDA Texture | Bulk Density (g/cm3) | Permeability (in/hr) | Available Water Capacity (cm3/cm3, in/in, or m/m) |
| Geary | 011 1134 3460 | Silty clay loam Silty clay loam/clay loam Silty clay loam/clay loam/silt loam | 1.301.40 1.351.50 1.301.40 | 0.62.0 0.22.0 0.62.0 | 0.180.23 0.170.20 0.150.19 |
| Hobbs | 08 860 | Silt loam Silt loam/silty clay loam/very fine sandy loam | 1.201.40 1.201.40 | 0.62.0 0.62.0 | 0.210.24 0.180.22 |
| Gothenburg | 04 460 | Sandy loam Sand and gravel | 1.401.50 1.701.90 | 2.06.0 > 0 | 0.130.22 0.020.04 |
| Boel | 012 1260 | Fine sandy loam Fine sand, loamy fine sand, coarse sand | 1.501.70 1.501.60 | 2.06.0 620 | 0.160.18 0.050.10 |
| Inavale | 06 618 1860 | Loamy fine sand Fine sand, loamy sand/loam fine sand Fine sand, loamy sand/loam fine sand | 1.501.60 1.501.60 1.501.60 | 620 620 620 | 0.100.12 0.060.11 0.050.10 |
| Valentine | 011 1160 | Fine sand Fine sand, loamy fine sand, loamy sand | 1.701.90 1.701.90 | 620 620 | 0.070.09 0.050.11 |
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| Figure 2.9. Wetting patterns early and late during a furrow irrigation water application. |
| Table 2.3. Example values of soil water characteristics for various soil textures.[a] | |||
| Soil Texture | ?fc | ?wp | AWC |
| cm3/cm3, in/in, or m/m | |||
| Coarse sand | 0.10 | 0.05 | 0.05 |
| Sand | 0.15 | 0.07 | 0.08 |
| Loamy sand | 0.18 | 0.07 | 0.11 |
| Sandy loam | 0.20 | 0.08 | 0.12 |
| Loam | 0.25 | 0.10 | 0.15 |
| Silt loam | 0.30 | 0.12 | 0.18 |
| Silty clay loam | 0.38 | 0.22 | 0.16 |
| Clay loam | 0.40 | 0.25 | 0.15 |
| Silty clay | 0.40 | 0.27 | 0.13 |
| Clay | 0.40 | 0.28 | 0.12 |
[a] Example values are given. You can expect considerable variation from these values within each soil texture. | |||
2.8 Infiltration
The reservoir of water in the soil is generally replenished by the process called infiltration, the entry of water through the soil surface. Infiltration is very important in irrigation since the goal is to supply water to the root zone to meet plant needs. In most cases, the goal is that all of the applied irrigation and rain enters the soil, thereby minimizing the amount of water that runs off the soil surface.
What causes water to enter the soil? Two things drive infiltration: capillarity and gravity. During the initial stages of a water application, capillary forces dominate water movement into the soil. Capillary forces work equally in all directions. Thus, capillary forces pulling water into the soil are the same in the horizontal and vertical directions. As time progresses, the capillary forces diminish, and gravity becomes the dominant force. This change in the dominant force is illustrated in Figure 2.9a where a wetted pattern under an irrigated furrow is almost semicircular in the early stages of an irrigation, but as infiltration progresses, the wetted pattern elongates in the vertical direction (Figure 2.9b). The elongation is due to the dominance of the gravitational force over capillary forces with time.
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| Figure 2.10. The rate of infiltration as an irrigation event proceeds and the steady rate of infiltration for three soil textures. |
Infiltration can be described in terms of either the rate of infiltration, which is the depth of water that infiltrates per unit of time, or the cumulative amount of water infiltrating over time. Cumulative infiltration is the total depth that has infiltrated after a specific time has elapsed. The curves shown in Figure 2.10 illustrate infiltration rates with time for several soil types. This figure applies where the soil surface is ponded instantaneously as would be the case for surface irrigation (i.e., furrows, borders, and basins). The curves show that initially the infiltration rate is very high and as time progresses, or more correctly, as the amount of water that has infiltrated increases, the rate of infiltration decreases. Therefore, a decay curve results with a decreasing rate of infiltration. As time continues, the infiltration rate will approach a nearly steady rate, sometimes called steady-state rate or basic infiltration rate or basic intake rate. Does the infiltration rate go to zero after a long period of application? No. It can only be zero if the soil is completely impermeable or if there is no gravity (outer space).
Cumulative infiltration, or the total depth of water infiltrated over time, is shown in Figure 2.11. The curves in Figure 2.11 show that cumulative depth increases with time, but it is not a straight line. Infiltration accumulates at a fast rate early and then slows later in the irrigation or rainfall event. The slope of the curve approaches the steady-state infiltration rate shown in Figure 2.10. Be careful not to confuse the soil cumulative infiltration or depth of infiltration with the depth to which water has penetrated in the soil. View it as you would water in a rain gauge. The depth of infiltration is analogous to the depth of water in the rain gauge. It is the volume of water that is infiltrated per unit of land area.
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| Figure 2.11. Examples of cumulative infiltration for three soil textures. |
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| Figure 2.12. Infiltration rate over time for a constant rate of water application that eventually exceeds the infiltration rate of the soil surface. |
What if ponding does not occur instantaneously such as with a gentle rain or with a stationary sprinkler system that has a constant rate of application? Initially, all of the water that falls from the rain or from the sprinklers will infiltrate the soil. However, if the application period is long, the intensity of the rain or the application rate of the irrigation system may exceed the infiltration capacity of the soil. When this occurs, water will pond on the surface (surface saturation). Once the surface layer is saturated, the infiltration rate begins to follow a curve similar in shape to the ponded water case. Figure 2.12 shows a situation where a stationary sprinkler system applies water at a rate which infiltrates initially, but then, at some point, surface ponding occurs. The irrigation system now is applying water at a rate faster than can be absorbed by the soil. From that time forward, the water not infiltrated is referred to as potential runoff. There is water on the soil surface, and after all the surface depressions are filled, runoff will begin. An ideal stationary sprinkler irrigation system would be designed so that the application rate does not exceed the steady-state infiltration rate of the soil. Thus, no runoff would ever occur. The ideal irrigation system, however, is rarely achieved.
For a moving sprinkler system, such as a center pivot or a traveling gun, the application pattern would appear similar to that shown in Figure 2.13. The application rate of the system increases with time as the irrigation system approaches a given location until it reaches a peak or maximum, after which it begins to decrease. It creates a symmetrical application rate versus time relationship in the absence of wind. For center pivots, the maximum occurs when the lateral pipe is directly above the given location. As was the case with the stationary sprinkler system, there may be a time when the soil can no longer absorb the water as fast as it is being applied. When surface ponding occurs, the rate of infiltration into the soil decreases as it did in Figure 2.11. Again, the difference between the infiltration rate curve and the system's application rate curve is potential surface runoff.
 |
| Figure 2.13. Infiltration rate as a function of time for a moving sprinkler system. |
| Table 2.4. Basic or steady-state infiltration rates for stationary sprinkler systems (adapted from Pair, 1983). | ||
| Soil Texture | Minimal Surface Sealing (in/hr) | Some Surface Sealing(in/hr) |
| Coarse sand | 0.751.00 | 0.400.65 |
| Fine sand | 0.500.75 | 0.250.50 |
| Fine sandy loam | 0.350.50 | 0.150.30 |
| Silt loam | 0.250.40 | 0.130.28 |
| Clay loam | 0.100.30 | 0.050.25 |
What factors influence the infiltration rate of the soil? Often, the first thing that comes to mind is the soil texture. We generally think of coarser-textured (sandy) soils having higher infiltration rates than fine- (clay) and medium-textured (loam) soils (Figures 2.10 and 2.11). Table 2.4 shows typical steady-state infiltration rates that can be expected for various soil textures. In theory, if the soils were uniform with depth, and if surface sealing did not occur, the steady-state infiltration rate would be equal to the permeability or saturated hydraulic conductivity of the soil. Permeability is a measure of a soil's ability to transmit water while saturated. The ranges of permeabilities of soils are often listed in soil survey reports (Table 2.2). Usually, ideal conditions do not exist in the field and, hence, various factors reduce the steady-state infiltration rate significantly below the permeability of the soil.
A major factor affecting infiltration is the method of water application. Infiltration is, in general, higher when the entire surface is wetted compared to only a portion of the surface. Thus, the infiltration rate (volume of water per unit of land area per unit of time) is generally higher for border and basin irrigation than it is for furrow irrigation, because with irrigated furrows the entire soil surface is not in contact with water.
Surface sealing is another factor influencing infiltration rate. Surface sealing occurs in both surface and sprinkler irrigation. With surface irrigation the shearing effect of the flowing water causes the soil aggregates on the surface to decompose into smaller aggregates and individual particles which tend to form a thin layer with low permeability on the soil surface. It is common to find large differences between infiltration during the first irrigation event and infiltration during later irrigation events due to surface sealing.
With sprinkler irrigation and rainfall, surfacing sealing is caused by the impact of the falling water drops on any exposed soil aggregates. Again, the aggregates are broken into smaller aggregates and individual particles, thus, forming a surface seal.
Another factor that has a large influence on infiltration is soil cracking. Soils that contain fine soil particles (clays) shrink when drying and swell during wetting. The shrinking soil cracks as it dries. These cracks cause the initial infiltration rate to be high as water flows freely into them. As the soil wets, the clay particles swell and the cracks close, which causes a rapid decrease in the infiltration rate.
Tillage also has a large impact on infiltration rate and, in fact, is often performed to enhance the infiltration rate. Conservation tillage practices that leave crop residues on the soil surface can also enhance infiltration. Crop residue on the surface protects the soil from the impact of water drops from sprinkler irrigation or rainfall, thus, reducing the formation of a surface seal. Likewise, deep tillage (chiseling) is sometimes used to enhance infiltration.
Soil water content is another factor that influences infiltration. The wetter the soil, the lower the infiltration rate. The initial infiltration rate of a moist soil is, in general, lower than the initial infiltration rate of an identical dry soil. As time progresses, the infiltration rate of these two conditions will converge to the same steady-state value.
Water temperature is also known to influence infiltration rates because temperature changes the viscosity of water. As temperature increases, the viscosity decreases, hence, the infiltration rate increases (Duke, 1992). The influence of this factor is often noticed where relatively cool groundwater is applied at the head of a surface irrigated field and begins to advance across the field. On a hot day, as the water moves across the field, it is warmed. As the water warms, the infiltration rate can go up and, thus, create a differential between the infiltration rate at the inlet end and the downstream end of the field. Sometimes this phenomenon is noticed when water in furrows that has already advanced to nearly the end of the field during the early, cool part of a day may actually recede back up the field as the day progresses. This observation is often incorrectly associated with a higher rate of evaporation from the water surface. In reality, the increased water temperature has increased the infiltration rate.
2.9 Storage of Infiltrated Water
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| Figure 2.14. Simplified storage of water infiltrating into a soil profile with three layers. |
Where does the water go once it has infiltrated into the soil? How deep will it penetrate into the plant root zone? Will it penetrate beyond the root zone?
Although an oversimplification, water applied to a soil can be viewed as filling the soil profile in layers as illustrated in Figure 2.14. Even if a soil layer is wetted to saturation, it is assumed that it quickly (in a few days) drains to field capacity. The excess water (excess of FC) from a soil layer drains to the layer immediately beneath it. This sequence continues until all of the water has been stored or reaches the groundwater. Within a few days, water in each layer drains to field capacity or a lower water content.
Water that penetrates deeper than the root zone is referred to as deep percolation. One goal in irrigation management is to minimize the amount of deep percolation. Deep percolation means that more water has been applied than necessary. Deep percolation transports chemicals below the root zone, a process called leaching. Leaching is generally unwanted but is sometimes required to remove excess salts from the root zone (as will be discussed in Chapter 7).
2.10 Measuring Soil Water Content and Matric Potential
Measuring soil water content and matric potential is important in irrigation management. Measuring soil water content is useful for determining whether soil water content is being kept within allowable bounds (to be discussed in Chapter 6), when the next irrigation should occur, and how much water the soil can hold without deep percolation. Many methods are available for measuring soil water content. We will only discuss a few of the more proven methods. For more detailed discussions of soil water measuring devices, refer to Evett (2007), Gardner (1986), or Ley (1994). This section does not discuss systems for logging soil water data, transmitting data to the cloud, data storage, or platforms for viewing and interpreting data. These systems make it much easier to incorporate soil water sensor data into day-to-day farm management, and many options are available from industry. A list of questions to consider when selecting a soil water monitoring system has been developed by ITRC (2019).
2.10.1 Gravimetric Method
The gravimetric method is the standard for measuring soil water content. By standard, we mean that it is often used to verify or calibrate other methods. This does not mean that it is the most frequently used method for irrigation management. Because of its high labor requirements, it is not used regularly. The procedure begins with taking a soil sample using a soil probe, soil auger, or shovel. Sample size should be at least 100 g (Ό pound). The soil is then sealed in an airtight container (frequently a plastic bag) so that moisture is not lost before weighing. Next, the wet mass of the sample is measured with a balance or scale that can be read with an accuracy of 0.5 grams (0.004 ounces). The sample is then dried at 105°C (220°F) for 24 hours in a forced air (preferable) or convection oven. Following drying, the sample is reweighed. Mass water content (?m) is determined by dividing the weight of the water by the weight of the dry soil. To determine volumetric water content (?v), the bulk density of the soil must be known (Section 2.3).
Gardner (1986) describes a method using a microwave oven for drying, which is helpful when results are needed quickly. The drying time is dependent on the initial water content and sample size. A typical drying time ranges from 10 to 30 minutes. A precaution is that a rapid rise in temperature occurs in the sample once the moisture has been driven out. If the temperature gets too high, some organic matter may burn which, in the calculations, might be erroneously mistaken for water loss.
Since sampling locations are not fixed or permanent, the gravimetric approach has the advantage that soil samples can be taken at any desired location within the field or irrigated area each time sampling occurs. Also, the method can give an accurate measure of volumetric water content, within 1%, if bulk density is known and a reliable balance is used for weighing. A disadvantage is the labor required to take the soil samples, especially at deeper depths. Another disadvantage is that the results are not immediately available.
2.10.2 Feel and Appearance
The feel and appearance method also requires the collection of soil samples at the desired depths. The soil sample is crumbled into small pieces and then squeezed by hand to form a ball. The cohesiveness of the ball is an indication of the soil's wetness. Also, whether it leaves an imprint in the palm of the hand after squeezing should be noted. The soil is then ribboned out between the thumb and the forefinger. Table 2.5 provides a detailed explanation of how to interpret the soil water content by the feel and appearance method.
This method requires a great deal of judgement and experience for good estimates of soil water. Nevertheless, it is widely used. Experienced users probably achieve an accuracy of fr plus or minus 0.10. Thus, if estimated fr = 0.55, the true value probably ranges from 0.45 to 0.65. This method is low in cost and allows moisture measurements to be taken quickly at multiple locations in the field. Considering the spatial variability of soil water in a field, the method can be adequate for irrigation management, especially if measurements are checked against a more accurate method periodically. A major disadvantage of this method is the need for experience before confidence is gained and accuracy is achieved.
| Table 2.5. Guide for judging how much water is available for crops (taken from USDA, 1972). | ||||
| Fraction of Available Soil Water Remaining | Feel or Appearance of Soil | |||
| Loamy Sand or Sand | SandyLoam | Loam andSilt Loam | Clay Loam orSilty Clay Loam | |
| 0 Wilting point | Dry, loose, single grained, flows through fingers. | Dry, loose, flows through fingers. | Powdery dry, sometimes slightly crusted but easily broken down into powdery condition. | Hard, baked, cracked, sometimes has loose crumbs on surface. |
| 0.25 | Appears to be dry, will not form a ball with pressure. | Appears to be dry, will not form a ball. | Somewhat crumbly but holds together from pressure. | Somewhat pliable, will ball under pressure. |
| 0.50 | Appears to be dry, will not form a ball with pressure. | Tends to ball under pressure but seldom holds together. | Forms a ball somewhat plastic, will sometimes slick slightly with pressure. | Forms a ball, ribbons out between thumb and forefinger. |
| 0.75 | Tends to stick together slightly, sometimes forms a very weak ball under pressure. | Forms weak ball, breaks easily, will not slick. | Forms a ball, is very pliable, slicks readily. | Easily ribbons out between fingers, has slick feeling. |
| 1 Field capacity | Upon squeezing, no free water appears on soil but wet outline of ball is left on hand. | Upon squeezing, no free water appears on soil but wet outline of ball is left on hand. | Upon squeezing, no free water appears on soil but wet outline of ball is left on hand. | Upon squeezing, no free water appears on soil but wet outline of ball is left on hand. |
| Note: Ball is formed by squeezing a handful of soil very firmly. | ||||
2.10.3 Neutron Scattering
An accurate method for measuring soil water is the neutron scattering or attenuation technique, which uses an instrument called a neutron probe. With this method, a radioactive source is lowered into an access tube installed vertically into the soil (Figure 2.15). The source is lowered to the desired depth of measurement and emits neutrons traveling at high speed. The speed of the neutrons is attenuated or slowed by hydrogen ions present in soil water. The rate of attenuation is dependent on the amount of water present. A detector, located near the source, counts the number of slow-moving neutrons over a short count period, 30 seconds to 2 minutes. There is a good correlation between the count of slow-moving neutrons and ?v.
An advantage of this method is the size of the soil volume sensed by the instrument. In effect, the probe samples a sphere with a diameter of 6 to 10 in, depending on soil water content. Neutron probes are also more accurate (within 1%) than most other soil water sensors. Disadvantages of the method include: (1) high initial cost, (2) a license is required to operate an instrument that is radioactive, (3) a calibration curve (Figure 2.16) must be developed for a given access tube material (usually aluminum, steel or polyvinyl chloride plastic) and for the soil of interest, (4) measurements within the top 6 to 8 in of soil are not reliable and require a separate calibration, and (5) measurements can only be made where the access tubes have been installed. The last item can be an advantage if repeated measurements at the same location in the field are desired. Neutron probes are often used in irrigation research.
 |
| Figure 2.15. Neutron attenuation method for measuring volumetric water content in a soil profile. |
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| Figure 2.16. Calibration curve for a neutron probe. |
2.10.4 Time Domain Reflectometry
One soil water measurement technique that takes advantage of the fact that a soil's apparent dielectric permittivity (ea) is dependent on ?v is time domain reflectometry (TDR). TDR requires the placement of two parallel rods (wave guides) into the soil. An electromagnetic wave is pulsed along the wave guides. The reflected signal from the tip of the wave guide is captured with a fast oscilloscope, recording voltage as a function of time. The travel time of the recorded wave must be calculated with a graphical interpretation of the waveform (with software) as part of the TDR method (Evett, 2007). The travel time provides a direct measurement of ea. The wave will travel faster in a dry soil than in a wet soil, with a lower travel time and a lower ea. The ea is comprised of the permittivity of the water, the permittivity of the soil, and the permittivity of air, and the water has a much larger influence on ea than the soil or air. Therefore, ea is directly proportional to ?v. Because of the strong correlation between ea and ?v, TDR is an accurate method for sensing ?v (within 2%). Its use was initially limited to research due to high costs, but ongoing technology development is reducing the price of TDR sensors. It has the advantage of not using a radioactive source, so licensing is not required. The measurement volume is approximately cylindrical and is dependent on the length of the rods and the spacing between rods. The diameter of the cylinder is about 1.5 times the spacing between rods.
2.10.5 Capacitance Probes
Similar to TDR, capacitance probes also take advantage of the correlation between ea and ?v. However, instead of measuring ea directly with travel time, it is estimated indirectly by quantifying capacitance and frequency, which is why these sensors are known as capacitance probes (or frequency domain reflectometry). This methoduses the soil as a dielectric and measures the capacitance of the soil (Evett, 2007). The capacitance circuit is pulsed with high-frequency radio waves. A natural resonant frequency is established which is dependent on the capacitance. The measured frequency is used to calculate the capacitance, which is used to determine the ea, which is correlated to ?v. Capacitance probes can be an easy-to-use option for monitoring trends in ?v; however, for accurate determination of the magnitude of ?v, capacitance probes are highly dependent on a calibration for the specific soil in which it is installed.
There are two forms of capacitance probes. One form has two or three electrodes which are inserted directly into the soil. The probe can be permanently installed at the desired depth in the soil profile, or it can be a portable device with the electrodes inserted at the soil surface. The measurement volume is dependent on the length and spacing of the electrodes. The second form requires an access tube (Rudnick et al., 2016), similar to neutron scattering. This allows soil water to be determined at multiple depths in the soil profile; however, the sensing volume is much smaller since the sensor is not in direct contact with the soil. Several types of capacitance probes are produced by industry for irrigation management.
2.10.6 Tensiometers
 |
| Figure 2.17. Components of a typical tensiometer. |
Soil water tension (matric potential, ?m) can be measured by several methods. The oldest tool, and one that measures tension directly, is the tensiometer. Tensiometers (Figure 2.17) have three components: a water filled tube (usually transparent); a porous cup (usually ceramic) at one end of the tube; and a vacuum gauge (or manometer) at the other end. The tube is sealed at the gauge end. The tensiometer is installed in the field so that the porous cup is at the desired soil depth. The cup must have direct contact with the surrounding soil so that the water in the cup is hydraulically connected to the water in the soil. As the soil dries, water is pulled out of the tensiometer. Since the tube is sealed at the gauge end, vacuum increases in the tube as water is being pulled out. Flow continues until there is equilibrium between the water in the tensiometer and the soil water. The vacuum gauge is a direct indicator of soil water tension. Usually, the vacuum is registered in centibars (cb) and the scale reads from 0 to100 cb. As the tension or vacuum approaches 1 bar, dissolved air in the water is released. The air accumulates in the top of the tube. When this happens, the readings are no longer reliable. Thus, the practical operating range for this instrument is 0 to 75 cb. A zero reading corresponds to a saturated soil, while a reading of 8 cb corresponds to FC for fine sand soils and a reading of about 20 cb is FC for silt loam soils, as shown in Figure 2.6. By using Figure 2.6, you should be able to demonstrate that, for fine sand, about 70% of the AWC has been depleted at 75 cb (the upper limit of the instrument), but only about 45% of the AWC has been depleted for silt loam at 75 cb. As will be discussed in Chapter 6, a common criterion for irrigation is to allow up to 50% depletion of the AWC before irrigation. This criterion indicates why the tensiometer has some limitations for irrigation management on finer-textured soils.
2.10.7 Electrical Resistance Blocks and Granular Matrix Sensors
Electrical resistance blocks consist of a porous material, usually gypsum, with two embedded electrodes (Figure 2.18). The blocks are buried at the desired soil depth. As with tensiometers, good contact with the surrounding soil is essential. When the soil water equilibrates with the water in the block, an ohmmeter with an AC current source can be used to measure electrical resistance between the electrodes. There is a relationship between the measured resistance and the water content of the gypsum, and the water tension in the gypsum is equal to the water tension in the soil. Therefore, the soil water tension (?m) and the measured electrical resistance are related. You might ask, why not just embed the electrodes directly into the soil and bypass the use of the gypsum? The problem with this approach is the effect of electrolytes in the soil on the resistance. Thus, electrical resistance in the soil is dependent on both soil water and soil salinity. The gypsum somewhat buffers the effect of the salts in the soil on observed resistance. In saline soils the effect of salts on the measured resistance cause inaccurate estimates of matric potential.
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| Figure 2.18. Various sensors for measuring soil water tension (from left to right): a gypsum electrical resistance block, a granular matrix sensor, a tensiometer, and a tensiometer installed in the soil. |
Gypsum blocks have largely been replaced by granular matrix sensors. One limitation of resistance blocks is that the gypsum matrix is a very fine material. Thus, the usable range is limited to high soil water tensions, usually greater than 50 cb. To overcome the limitation of gypsum blocks to the wet range, blocks composed of a coarser media, such as sand, have been developed. These coarser blocks, referred to as granular matrix sensors, have a usable range of 5 to 200 cb (Evett, 2007). Granular matrix sensors have a longer usable life than resistance blocks. Another advantage is that granular matrix sensors are low cost compared to most other soil water sensors. The low cost makes it possible to install a large number of sensors in a field, in order to better account for spatial variability in soils. Also, on a small scale (cm to m), the spatial variability in ?m is somewhat lower compared to ?v, so a measurement of soil water tension may represent a larger volume of soil than a ?v sensor with a relatively small measurement volume. A disadvantage is that, if ?v is desired, a soil water release curve is needed to convert ?m to ?v, which introduces more uncertainty along with the normal uncertainty from soil water sensor data. For this reason, irrigation scheduling based on granular matrix sensors often uses ?m directly, comparing it to a threshold ?m where crop stress would be expected to occur. University extension guides have been developed with specific guidance on using granular matrix sensors (Irmak et al., 2016).
2.10.8 Thermal Dissipation Blocks
Another, less common, approach that uses porous blocks has a heater and temperature sensor embedded within the block. The porous block installation must result in good contact with the soil, allowing water tension in the block to come into equilibrium with the water tension in the surrounding soil. The block is heated by passing current through the heater. The rate of heat dissipation in the block is then measured. The rate of heat dissipation is directly related to the water content in the block, and, since the porous block has a known release curve, the water content of the block is directly related to water potential in the block and soil. An advantage of heat dissipation blocks is that they are sensitive to soil water over a wide range. Unfortunately, the heat dissipation blocks must be individually calibrated and they are considerably more expensive than granular matrix sensors. A potential application of this concept is using heated cables to determine water content at many locations along the cable (Sayde, 2010). Since the heat dissipation occurs in the soil (instead of a block), heat dissipation would be correlated to ?v instead of ?m. This method is still under development.
2.10.9 Placement of Soil Water Sensors
The above methods of soil water measurement require that representative sites be selected for sampling. This means that sampling must consider the variability in soils, the variability of water applications, and the variability of plant populations within the irrigated area. The microclimate around the area to be measured should also be considered. This is especially important in landscape applications where buildings and streets can greatly affect the environment surrounding the irrigated plants.
Soil water measurements must be taken at depths that represent the plant root zone. Estimates of soil water content secured with the heel of your boot usually are inadequate to describe what is really happening within the plant's root system. In Chapter 6, root zone depths for various crops will be presented. Installation of soil water sensors should be done with great care, since a good installation is required in order to obtain high-quality data.
One of the frustrations of measuring soil water is the large number of samples that are required before you feel comfortable with how well the measurements represent the soil water conditions in the irrigated area. Because of natural variability of soil properties and the variability in depth of rainfall and irrigation applications within the irrigated area, considerable variability in measured soil water can be expected. Another problem is the number of locations that must be monitored to truly represent the plant root zone. A minimum of two soil depths should be measured, and often three or four are required to properly represent root zone soil water conditions. One approach to reduce the uncertainty in soil water data is to focus more on the trends over time rather than the magnitude of the measurement. If possible, determine the FC from the sensor data after it is installed (looking at the trend after a large wetting event which saturates the soil profile), and track the amount of depletion below FC (i.e., calculate SWD) over time.
2.10.10 Remote Sensing
Ongoing research is investigating remote sensing as a possible method to determine ?v for irrigation management. Generally speaking, this could include satellite remote sensing, airborne remote sensing (manned or unmanned aircraft), and proximal remote sensing (sensor placed near the crop). For above-ground, moving irrigation systems, proximal remote sensing can include sensors mounted on the irrigation system itself (e.g., mounted on the lateral of a center pivot). The primary advantage provided by remote sensing is the spatial dataset, allowing the user to quantify spatial patterns in soil water. The primary limitation is that measurement of ?v with remote sensing is typically restricted to a shallow layer of soil at the soil surface (i.e., top 10 to 25 cm), which can be problematic if using irrigation to manage the entire root zone for deep-rooted crops (e.g., 100 cm). One proximal technology is the cosmic-ray neutron probe (Franz et al., 2015); ongoing research is estimating root zone ?v based on shallow ?v from the cosmic-ray neutron probe (Franz et al., 2020). Microwave remote sensing can also be used to estimate ?v, with sensors mounted on a center pivot (Qiao et al., 2016). The Soil Moisture Active Passive (SMAP) satellite (U.S. National Aeronautics and Space Administration) and the Soil Moisture and Ocean Salinity (SMOS) satellite (European Space Agency) are examples of satellites which are used to produce soil water data products (Al-Yaari et al., 2019).
2.11 Summary
One of the most important functions of a soil is to serve as a reservoir for storing precipitation and irrigation water for use by plants. Water is stored in the void spaces between soil particles. When the voids are filled with water, the soil is said to be saturated. A saturated soil rapidly drains to a, more or less, constant moisture level called field capacity (FC). Plants can extract water from the soil until the soil water content reaches the permanent wilting point (WP). At permanent wilt, plants will not recover even if the soil is rewet. The difference between the water content at field capacity and wilting point is called available water capacity (AWC). Finer-textured soils have a higher AWC than coarser-textured soils. AWC ranges from 0.05 to 0.25 inches of water per inch of soil.
The equivalent depth of water stored in a soil layer of known thickness can be determined if the volumetric water content (?v) is known. Total available water capacity (TAW) of the root zone is the product of the root zone depth and the AWC. Soil water deficit (SWD) is the amount of water that has been depleted from the TAW. Allowable depletion (AD) is the maximum soil water deficit that should occur before water is applied. Plant water stress will occur if SWD exceeds AD.
Water enters the soil by infiltration which is due to capillary and gravitational forces. The process is affected by the method of irrigation. When cumulative infiltration exceeds SWD, water percolates below the reach of plant roots and, simultaneously, leaches dissolved salts and chemicals from the root zone.
Soil water content can be determined by a variety of methods. The simplest and least accurate is by feel. The standard method is collecting soil samples and weighing them before and after oven drying. There are a number of sensors or devices that are buried in the soil from which readings are made to infer soil water content or soil water potential.
Questions
1. Would a fine-textured soil have a higher or lower available water capacity than a coarse-textured soil? How would the bulk densities compare? Explain why the differences, if any, occur.
2. Describe total soil water potential and its components. Why is matric potential important in irrigation management?
3. Will water infiltrate into the soil even if the root zone is at field capacity? If so, where will the water be stored? Explain.
4. Repeat Question 3 for a saturated soil.
5. Show mathematically how Equation 2.13 is derived from Equation 2.12.
6. The use of wetting agents has often been suggested to enhance infiltration. Wetting agents act by reducing the surface tension of the liquid. What effect does this have on capillary forces and infiltration? Explain.
7. An irrigation of 2.5 in of infiltration is followed by 1 in of rainfall infiltration. If a clay loam soil had a 50% depletion of the available water (fd = 0.5) prior to the water application and the root zone depth is 30 in, how much water would deep percolate?
8. If the average count ratio for neutron scattering measurements was 1.0, how much water needs to be infiltrated to bring a silt loam soil to field capacity?
9. A soil to be irrigated has two layers: the top layer is a silt loam 12 in deep and the other is a silty clay with a thickness of 3 ft. If both layers were at the permanent wilting point, how much water could be applied without water draining below the first layer? Second layer?
10. If you install a granular matrix sensor into the silt loam soil depicted in Figure 2.6 and the reading was 35 cb, what would be the volumetric water content of this soil? If the bulk density of the soil was 1.35 g/cm3, what would be the mass water content?
11. If the average count ratio from a series of measurements with the neutron probe whose calibration is shown in Figure 2.16 on a golf course was 0.5 before irrigating and 0.9 after irrigating, how much water was added to a soil profile 1 ft deep?
12. A soil sample was taken just prior to irrigation and weighed wet then dried and re-weighed.
The following data were obtained:
Wet mass = 240 g
Dry mass = 200 g
The soil has the following characteristics:
?fc = 0.30 cm3/cm3
?wp = 0.10 cm3/cm3
?b = 1.25 g/cm3
Determine the following:
?m = mass water content
?v = volumetric water content
fr = fraction of available water remaining
fd = fraction of available water depleted
13. Tensiometers are placed in a fine sandy loam soil at depths of 6, 18, and 30 in. The following readings were taken:
| Depth (ft) | Tensiometer Reading (cb) |
| 6 18 30 | 30 70 50 |
Use Figure 2.6 to help answer the following questions. Assume each tensiometer reading represents 1 ft of soil.
a. Determine the available soil water remaining at each depth (in/ft).
b. Determine the total available soil water remaining, in inches, in the 3-ft profile.
c. What is the fraction depleted in each layer?
d. How much water would have to be applied to bring the soil water level to field capacity to a depth of 3 ft?
14. Answer the following:
a. Using the soil water release curves shown in Figure 2.6, determine the expected soil moisture tensions at fd = 0.3 and fd = 0.6 for the silt loam soil.
b. Repeat (a) for the fine sand.
c. Would a tensiometer work properly for the four cases in (a) and (b)? Explain your answer.
15. The feel and appearance method was used to estimate the soil water in the following layers:
| Depth (ft) | fr |
| 01 | 0.55 |
| 12 | 0.65 |
| 23 | 0.60 |
| 34 | 0.80 |
| 45 | 0.80 |
| 56 | 0.80 |
| 67 | 0.80 |
a. If the volumetric water content at field capacity and wilting point is 0.35 and 0.13, respectively, how deep would 4.2 in of infiltrated water penetrate into the soil profile? Give your answer in inches.
b. If the root zone depth is 30 in, how many inches of water would percolate below the root zone?
References
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