Estimation of Net Radiation and Evapotranspiration in California Using MODIS Satellite Observations

Soil moisture links surface energy, water and biogeochemical cycles by several different pathways, including by influencing the partitioning of energy into latent and sensible heat and by regulating NPP and heterotrophic respiration fluxes. Evapotranspiration (ET) is a major pathway for water loss and its seasonal variation affects the seasonality of soil moisture and subsequently net ecosystem exchange. We developed an empirical ET algorithm using Ameriflux data and MODIS leaf area index to improve the estimation of soil moisture in CASA biogeochemical model at a regional scale. We estimated net radiation (Rn) using MODIS BRDF/albedo and skin temperature/emissivity products. A good agreement was found between satellite-based estimates and field- measured Rn from SURFRAD, Ameriflux, and 6 recently installed flux towers in southern California, with an absolute difference below 30 W m-2. The ground heat flux component of available energy was estimated using the fraction of vegetation derived from MODIS NDVI. We parameterized the Priestly-Taylor coefficient with leaf area index and soil moisture at an 8-day time scale using multi-year data from the Ameriflux sites. We validated this algorithm using ET measurements from May 2006 to April 2007 in southern California tower sites. The spatial distribution of annual mean Rn over California showed an increasing trend from desert to grassland ecosystems, and from grasslands to forests, reflecting decreasing albedo and surface temperature with increasing vegetation cover fraction. The Priestly-Taylor coefficients followed the phenology and the seasonality of soil moisture reasonably well, which leads to higher ET in spring rather than in summer- when Rn peaks. The seasonal cycle of net ecosystem exchange predicted by CASA with these improvements agreed reasonably well with those derived from California's eddy covariance measurements due to the improved seasonality of soil moisture.