Methods for Estimating the Evaporation from Shallow Water Table in Drainage Design Formulas

Abstract

The evaporation plays an important part in lowering the shallow groundwater table. In many areas this factor should not be ignored in subsurface drainage design. Empirical formulas for estimating steady evaporation from shallow water table derived on the basis of lysimeter data have long been used by Averianov (1956), Hammad (1962), Zhang (1963) and others. In this paper an approximate method for determining evaporation from groundwater based on the empirical relationship and groundwater regime observation data is described. Theoretical formulas for computing steady evaporation from water table have been developed by Gardner (1958) for homogeneous soil. Willis (1962) used these formulas to determine the evaporation in two-layered soil by curve-matching. We extended the use of steady solutions to multilayered soils (Y. Zhang 1966). Experiments and theoretical study shows that the evaporation depends upon not only the climatic conditions (usually characterized by potential evaporation), but also the capacity of soil to transport water from groundwater to soil surface. The evaporation can never exceed the smaller value between the above two limitations. In this paper graphs based on experimental data showing the above relationship are given. Under the condition of subsurface drainage the water table is always descending, hence the evaporation is also changing with time. As the steady evaporation is the maximum value for a given soil at a definite depth to groundwater and under definite climatic conditions, when this value is used in design the effect of drainage will be overestimated. With the aim of clarifying the process of evaporation during drainage, in this paper through numerical simulation the nonsteady evaporation under typical patte m of drawdown of water table for some soils is investigated.