Abstract

Comparison of Actual Evapotranspiration Estimates to a Soil Moisture Budget and Plant Growth

Presented at the 4th Annual Washington Hydrogeology Symposium, April 8–10, 2003, by Erick Miller, PG; Peter Bannister; Louis Licht, PhD, PE; and Anne Holmes

Evapotranspiration is an integral part of a hydrologic water balance. Actual evapotranspiration (AET) is the combined loss of water to the atmosphere through evaporation from the soil surface, evaporation of intercepted water, and plant transpiration.

The standard approach to estimating AET is calculation of a reference-crop potential evapotranspiration (PET0), assuming soil moisture is not limited. Grass is a widely adopted reference evaporating surface. Calculations of PET0 are dependent only on climate parameters, without influence from soil parameters (ASCE Manual no. 70, FAO 56). The Penman-Monteith method was adopted as the standard for computing PET0 in FAO 56. Input parameters include solar radiation, relative humidity, temperature and wind speed. Crop-specific potential evapotranspiration (PETC) is related to PET0 by multiplying by a seasonally variable crop coefficient (KC). Adjustments are made to PETC to account for evaporation of water intercepted by the crop canopy.

AET equals PET when available soil moisture is not limiting — that is, until the matric potential is greater than the roots’ ability to extract soil moisture. This point typically occurs when soil moisture is near the midpoint between field capacity and the wilting point (FAO 56). For lower soil moisture values, AET declines linearly to zero at the wilting point.

An intensive study of a poplar tree cover planted on a landfill near Duvall, Washington, yields data for comparing AET estimates from grass reference-crop PET and soil moisture with results from 1) soil moisture budgets at multiple locations and 2) water uptake estimates based on plant growth. The soil moisture budget assigns volumetric soil moisture measured by time domain transmissometry (TDT) or frequency domain reflectometry (FDR) probes at three depths to a representative length of the soil column. Losses in soil moisture during periods of no precipitation or runoff were compared to AET computed using the FAO 56 method. Plant growth was measured before and after a growing season, and estimates of total water uptake were estimated and compared to season total AET computed using the FAO 56 method. The comparisons provide empirical evidence to ground-truth evapotranspiration estimates based on climatological data.