Abstract:
Evapotranspiration (ET) is the sum of evaporation from the soil surface, transpiration through plant stomata, and evaporation from the wet canopy surface. ET is a key hydrologic variable, linking the Earth’s water, carbon and energy cycles. Additionally, ET estimates are used to monitor agricultural water use and drought conditions, making accurate global ET estimates a necessity. The NASA MODIS MOD16 algorithm is an operational satellite ET product, which is available globally at relatively fine spatial (500m) and temporal (8 day) resolutions. However, despite the strong mechanistic foundations of MOD16, the current algorithm lacks a soil moisture control on ET, which can reduce model accuracy.

To address this issue, we adapted the MOD16 framework to use satellite microwave soil moisture retrievals as an additional constraint on the ET calculations. The updated ET algorithm uses MODIS surface reflectance and NASA Soil Moisture Active Passive (SMAP) mission Level 4 surface and rootzone (0-1m depth) soil moisture as key inputs to estimate the daily soil and stomatal conductance terms of the algorithm. The MOD16 environmental response parameters were calibrated using Ameriflux tower eddy covariance ET observations representing major North American biomes. The model ET results were then validated using a larger set of tower ET observations spanning a large regional climate gradient. The updated ET estimates outperformed the baseline MOD16 product across all tower validation sites (RMSE = 0.74 and 0.94 mm/day, and R2 = 0.51 and 0.42, respectively). The model ET improvements are more pronounced in arid regions (RMSE = 0.53 and 0.65 mm/day, and R2 = 0.61 and 0.30, respectively), consistent with a stronger soil moisture control on ET in water limited regions. Our results show that the use of SMAP soil moisture observations as an additional model constraint improves MOD16 performance, while providing a new framework for investigating both soil and atmosphere controls on ET.