ESTIMATES OF NATURAL SOIL-DRAINAGE VOLUMES IN SOUTH DAKOTA

Abstract

Drainage water that would pass through successively deeper soil layers was estimated at selected weather stations. Monthly Thornthwaite potential evapotranspiration amounts were subtracted from the precipitation amounts to calculate the yearly net soil water addition, or loss if the soil lost plant-available water. Surplus plant-available water at year's end was the initial amount in the next year's calculation. With each monthly addition, the total depth of water penetration was calculated. For fine-textured clay soils, water drainage does not occur below 1 m in the northwestern quarter of the state, but in the southeastern corner, a total of 3.5 m of water drains below 1 m during 10 or 11 yr in a 100-yr period. For sandy soils, drainage does not occur below 3.0 m in the northwestern quarter, but in the southeastern corner up to a total of 4 m of water drains below 3.0 m during 10 to 13 yr in 100 yr. Plants with deep root systems, such as alfalfa, could remove subsoil water to this depth. Drainage of water from the soil profile is unlikely on nearly level upland areas that were originally prairie and that do not receive run-in water. Semiarid, northwestern South Dakota has high evapotranspiration and low precipitation, so that drainage through the subsoil is at least low if not negligible on nearly level upland landscapes. Precipitation and probably subsoil drainage increase southeast across South Dakota. The drainage would be used by plants if it were not deeper than the rooting zone. To assess leaching potential, weather and soil data were used to calculate the volume of drainage water that would pass through successively deeper soil layers in South Dakota.