Total Soil Water Evaporation in a Riparian Environment: Model Development and Application

Abstract

A total soil water evaporation model that includes both transient (event based) evaporation and water table evaporation was developed by modifying an existing water balance type model incorporated in the widely used FAO-56 method. Using results from a parametric numerical study, the method applies a predictive equation for soil water table evaporation as a function of distance to the water table, soil properties, and climatic conditions. The model, which can be easily implemented in a spreadsheet, can account for the effects of water management decisions relating to tree canopies and surface mulch. Applied to a location in the Middle Rio Grande riparian zone over the course of a year, the model provides insight into how soil water evaporation may be impacted by restoration and management activities. Soil hydraulic properties were shown to greatly affect the predicted amount of total soil water evaporation. With a relatively shallow water table and an exposed soil surface, the amount of soil water evaporation can be comparable to water consumption by riparian vegetation. Canopy shade reduces the amount of total soil water evaporation by more than 50%. As expected, the model indicates that surface mulch yields an even greater reduction in total soil water evaporation. Mulching bare soil surfaces can effectively reduce evaporative losses after exposing a soil surface by removing undesirable vegetation. This study shows that riparian tree removal in areas of shallow ground water may result in no water savings.