A model for estimating evapotranspiration of natural ecosystems

In recent years, the availability of near-real time and forecast standardized reference evapotranspiration (ET_o) has increased dramatically. Use of the ET_o information in conjunction with “landscape” coefficient (K_L) factors that adjust for differences between the vegetation and the reference surface provides a method to greatly improve landscape (or ecosystem) evapotranspiration (ET_L) estimates. Difficulties in estimating ET of well-watered vegetation in an ecosystem result from local advection and edge effects, wide 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 an ecosystem that is water stressed is even further complicated because of stomatal closure and reduced transpiration. The Ecosystem Water Program (ECOWAT) was developed to help improve estimates of 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 integrated fire danger systems. The first step in estimating ET_L is to use monthly climate data from a location that represents the region to estimate ET_o. Then, the topography and monthly local microclimate data are input and ET_o is again calculated to provide a standardized reference evapotranspiration for the local microclimate (ET_m). The topography information adjust the solar radiation for slope and aspect to estimate net radiation on slopes, which improves the ET_m estimates. The ratio K_m = ET_m/ET_o is calculated and applied as a microclimate correction factor to estimate ET_m. There are natural variations in evapotranspiration between plants, so the product of ET_m and a vegetation coefficient (K_v = ET_v/ET_m) is used to estimate the ET of the ecosystem vegetation (ET_v) under well-watered conditions at the same location. The ET_v and K_v information must be determined experimentally. Next, a coefficient for plant density (K_d) that is based on the percentage ground is used to adjust the full-canopy ET_v to the ET of a well-watered ecosystem (ET_w). Thus, ET_w = ET_o×K_m×K_v×K_d provides an estimate of the ecosystem ET assuming there is little or no transpiration-reducing water stress. In the ECOWAT model, a stress (K_s) coefficient is determined by calculating changes in soil moisture as water is lost to ET. The K_s factor varies between 1.0 with no stress to 0.0 with full stress. The K_s = 1.0 until the plants reach a designated percentage depletion of available water. Then, the K_s value decreases linearly from 1.0 at the critical soil moisture to 0.0 at the permanent wilting point. The critical soil moisture is calibrated by comparing with measured data for a particular ecosystem. The actual ecosystem (or landscape) evapotranspiration (ET_L) is estimated as ET_L = ET_w×K_s. In this paper, we will present how the ECOWAT model works and how it performs when the model outputs are compared with measurements taken in a Mediterranean maquis ecosystem over several years. In addition, the potential use of ECOWAT in integrated fire danger systems (e.g., IFI, FWI, etc.) will be discussed.