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The rate, amount and quality of water leaving agricultural land are to a great extent determined by the design and management of any drainage system. In this paper, recession curves were used to identify differences in storage-outflow relationships between two different subsurface drainage systems, conventional and controlled drainage. A three-year (1996-1999) field experiment was carried out on a loamy sand soil in southern Sweden. Plots with an area of 0.2 hectares were drained by conventional or controlled drainage, the latter allowing a certain control of the groundwater level in the soil during the year. Measurements were performed to record precipitation, drain outflow and groundwater levels. Selected recession curves of hourly measurement of outflow behaved like single reservoirs, and a linear storage-outflow model was applied. Least squares estimates of the parameters initial outflow (Q0), initial storage volume (S0) and retention constant (k) were calculated for each curve. For conventional drainage, Q0 ranged between 0.098 and 0.214 mm h-1, S0 between 7.2 and 15.5 mm and k between 39.7 and 103.8. For controlled drainage, Q0 ranged between 0.092 and 0.426 mm h-1, S0 between 1.0 and 6.0 mm and k between 4.7 and 29.8.

Controlled drainage had a large effect on total drain outflow and outflow pattern during the three years of measurement. The analysis revealed that Q0 became higher, and k and S0 lower when the groundwater level in the field was raised. The decrease in S0 resulted in a slower release of soil water between rainstorms due to less interflow and thereby a reduction in baseflow through the subsurface drainage system. The hydrological impacts of the reduction in S0 were higher peak flow, shorter lag time and shorter recession time and these effects increased with higher groundwater level. Thus, when there are obvious differences in groundwater level between similar sites, recession analysis could be a useful tool in revealing relative differences in storageoutflow relationships of small agricultural fields in order to predict downstream effects.