TR-119 : Prediction of evaporation losses from shallow water table using a numerical model

Abstract

A steady state flow problem of interest and importance is the upward movement of water from a water table and subsequent evaporation at the soil surface. This information is desirable when estimating water loss from soils by evaporation and estimating the amount of ground water available to plants due to the upward movement of water from a water table. Soils may also become saline due to the upward movement of saline ground water and its subsequent evaporation at the soil surface. To minimize the rate of salt accumulation and thus reduce the salinity hazard, attempts are usually made to lower the water table by pumping or by installation of drains. In order to determine what depth to water table should be maintained, the relation between depth to water table, soil properties, and evaporation rate must be known.

The purpose of this study is to estimate the steady state evaporation rates from bare soils under conditions of high water table. A finite difference numerical scheme based upon the one-dimensional Richards equation has been employed to attain the steady state moisture profiles and estimate the evaporation rates under conditions of high water table. The procedure takes into account the relevant atmospheric factors and the soil's capability to conduct water. Field data required include soil water retention curves, water table depth, and a record of air temperature and air humidity. Results obtained with the method demonstrate how the soil water evaporation rates depend on water table depth and suction prevailing at the soil surface.