Study of Soil Water Movement and Groundwater Recharge for the Loess Tableland Using Environmental Tracers

Groundwater is the primary water resource for human use on the Loess Tableland of China; however, the depletion of groundwater has been very severe in recent years. To understand the reasons for groundwater depletion, it is necessary to understand its recharge mechanisms. The objective of this study is to reveal the mechanisms of groundwater recharge on the Loess Tableland using deuterium and oxygen-18 in precipitation, in soil water under three land use types (natural grassland, wheat field, and apple orchard), and in groundwater. Results indicate that piston flow and preferential flow coexist in rainfall infiltration; however, their occurrences are greatly influenced by land use type. They both occurred in the natural grassland and wheat field; however, only piston flow was detected in the apple orchard. Groundwater is likely to be recharged by both piston flow and preferential flow through the unsaturated zone, while preferential flow should be the main recharge mechanism, as the isotopic composition of the groundwater is very close to that of precipitation while quite different from that of deep soil water. However, groundwater recharge by preferential flow only occurred in the natural grassland or wheat field, but not in apple orchard due to its thick dry soil layer. Regarding groundwater recharge by piston flow, it possibly occurs in most years for the natural grassland and in wet years for the wheat field; however, it is possibly difficult for the apple orchard to recharge groundwater by piston flow. The above results suggest that the conversion of farmland to apple orchard can affect the natural mode of the water cycle and reduce groundwater recharge on the Loess Tableland; therefore, attention should be paid to adjustments of land use.