Abstract
Abstract. With increasing crop water demands and drought threats, mapping and monitoring of cropland evapotranspiration (ET) at high spatial and temporal resolutions become increasingly critical for water management and sustainability. However, estimating ET from satellites for precise water resource management is still challenging due to the limitations in both existing ET models and satellite input data. Specifically, the process of ET is complex and difficult to model, and existing satellite remote-sensing data could not fulfill high resolutions in both space and time. To address the above two issues, this study presents a new high spatiotemporal resolution ET mapping framework, i.e., BESS-STAIR, which integrates a satellite-driven water–carbon–energy coupled biophysical model, BESS (Breathing Earth System Simulator), with a generic and fully automated fusion algorithm, STAIR (SaTallite dAta IntegRation). In this framework, STAIR provides daily 30 m multispectral surface reflectance by fusing Landsat and MODIS satellite data to derive a fine-resolution leaf area index and visible/near-infrared albedo, all of which, along with coarse-resolution meteorological and CO2 data, are used to drive BESS to estimate gap-free 30 m resolution daily ET. We applied BESS-STAIR from 2000 through 2017 in six areas across the US Corn Belt and validated BESS-STAIR ET estimations using flux-tower measurements over 12 sites (85 site years). Results showed that BESS-STAIR daily ET achieved an overall R2=0.75, with root mean square error RMSE =0.93 mm d−1 and relative error RE =27.9 % when benchmarked with the flux measurements. In addition, BESS-STAIR ET estimations captured the spatial patterns, seasonal cycles, and interannual dynamics well in different sub-regions. The high performance of the BESS-STAIR framework primarily resulted from (1) the implementation of coupled constraints on water, carbon, and energy in BESS, (2) high-quality daily 30 m data from the STAIR fusion algorithm, and (3) BESS's applicability under all-sky conditions. BESS-STAIR is calibration-free and has great potentials to be a reliable tool for water resource management and precision agriculture applications for the US Corn Belt and even worldwide given the global coverage of its input data.
Highlights
Accurate field-level management of water resources urgently demands reliable estimations of evapotranspiration (ET) at high spatial and temporal resolutions
In the global Breathing Earth System Simulator (BESS) ET product (Jiang and Ryu, 2016), these vegetation variables are derived from MODIS satellite data at 1 km resolution, while in BESS-SaTallite dAta IntegRation (STAIR) they are derived from 30 m resolution surface reflectance fused from high spatial resolution Landsat data and high temporal resolution MODIS data by STAIR
We evaluated vegetation indices (VIs)-based leaf area index (LAI) and radiative transfer model (RTM)-based LAI estimations derived from 30 m resolution STAIR fused surface reflectance data against field measurements
Summary
Accurate field-level management of water resources urgently demands reliable estimations of evapotranspiration (ET) at high spatial and temporal resolutions. In the US Corn Belt, where more than 85 % of corn and soybean is produced in the US (Grassini et al, 2015), increasing vapor pressure deficit (VPD) and drought sensitivity have been recognized as severe threats to future crop security (Lobell et al, 2014; Ort and Long, 2014). The vulnerability to drought in this region is further exacerbated by elevated rates of grass-to-crop conversion and expansion of irrigated areas (Brown and Pervez, 2014; Wright and Wimberly, 2013). Precision water resource management requires the capacity to account for spatial heterogeneity and to guide real-time decision-making (GAO, 2019). Reliable tools are urgently needed to estimate, map, and monitor the total amount and spatial and temporal variations of cropland ET
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