ECOWAT – theory and calibration

Standardized reference evapotranspiration (ETo) is commonly used in agriculture and urban landscape water management as measure of evaporative demand. Crop (Kc) and landscape (KL) factors are multiplied by the ETo to estimate evapotranspiration of crop or landscape vegetation. The coefficients adjust mainly for differences in net radiation and canopy and aerodynamic resistance differences between the ecosystem vegetation and ETo from the reference surface. Difficulties in estimating ET of well-watered vegetation in any ecosystem result from local and regional advection, variations in radiation resulting from undulating terrain, wind blockage or funneling, and differences in temperature due to spatial variation in radiation, wind, etc. Estimating the ET of a water stressed ecosystem is even further complicated because of effects on plant growth and stomatal closure. The Ecosystem Water Program (ECOWAT) was developed to estimate ET by accounting for microclimate, vegetation type, plant density, and water stress. In combination with GIS, the output from ECOWAT is a potential tool to improve hydrologic and fire danger models. Regionally representative monthly climate data are input to estimate mean daily ‘regional' ETo for each month. Then, measured or estimated microclimate data are input to estimate daily mean ‘local' ETo (ETm) by month, and a monthly microclimate coefficient (Km = ETm/ETo) is determined. A cubic spline curve fitting is used to estimate daily values for Km, which are used with near real-time ETo to estimate daily ETm. The product of ETm and a vegetation coefficient (Kv = ETv/ETm) is used to estimate the well-watered ET of the ecosystem vegetation (ETv) at the same location. The Kv coefficient is determined using micrometeorological data collected during periods when soil water is not limiting. Next, a coefficient for plant density (Kd) that is based on the percentage ground is used to adjust the full-canopy ETv to the ET of a well-watered ecosystem (ETw) of known density. A stress (Ks) coefficient is determined by calculating changes in soil moisture as water is lost to ET. The Ks factor varies between 1.0 with no stress to 0.0 with full stress. The Ks = 1.0 until the plants reach a designated percentage depletion of available water. Then, the Ks value decreases following a sine wave curve from 1.0 at the critical soil moisture to 0.0 at the permanent wilting point. The critical soil moisture is determined by comparing with measured data for a particular ecosystem. The actual ecosystem (or landscape) evapotranspiration (ETL) is estimated as ETL = ETw×Ks, but it is not allowed to fall below a baseline bare-soil evaporation rate that is estimated from the ETo rate and soil wetting frequency. In many ecosystems, the cumulative ETL rate indicates that the available soil water content is completely depleted; however, field measurements often indicate that evapotranspiration is still occurring. Based on a range of ecosystems and climates, we found that making small corrections for likely contributions from water tables, fog, and dew will often explain these discrepancies. In this paper, we will present the ECOWAT model theory and how it can be calibrated. In a second paper, we will present several examples of the model performance.