## 2 Data and Methods
### Uncoupled MODIS GPP and ET Products  
The MOD17A2 GPP and MOD16A2 ET products with an eight-day temporal resolution and a 500 m spatial resolution were downloaded from Level-1 and the Atmosphere Archive and Distribution System Distributed Active Archive Center (LAADS DAAC) ([[https://ladsweb.modaps.eosdis.nasa.gov/search/order](https://ladsweb.modaps.eosdis.nasa.gov/search/order)]([https://ladsweb.modaps.eosdis.nasa.gov/search/order](https://ladsweb.modaps.eosdis.nasa.gov/search/order)), accessed on 7 October 2023). The MOD17 GPP product relies on the light use efficiency (LUE) scheme (namely, the MOD17 model), which regards the GPP as the product of the photosynthetically active radiation (PAR), the fraction of photosynthetically active radiation (FPAR), the maximum LUE, and the environmental scalars [21]. Specifically, the PAR is calculated from the income shortwave radiation provided by the National Center for Environmental Prediction--Department of Energy (NCEP-DOE) Reanalysis II meteorology dataset; the FPAR is directly adopted from the MOD15A2 LAI/FPAR product; the maximum LUE is derived from a PFT-dependent look-up table; and the environmental scalars (representing the hydrological and thermal stresses on photosynthesis efficiency) are calculated by two piecewise linear functions of the vapor pressure deficit (VPD) and air temperature (Ta), respectively. Moreover, the MCD12Q1 land cover type product is used to identify the PFT of each pixel.  
The MOD16 ET product is generated by the improved ET algorithm [9] over the previous algorithm [22] based on the Penman-Monteith equation (namely, the MOD16 model). Modified from the Penman equation that assumes ET is controlled by the drying power of the atmosphere (i.e., VPD and wind speed) and the available energy [23], the Penman-Monteith equation further considers the surface control on evaporation and  
Figure 1: Satellite images over a grid area of 500 m \(\times\) 500 m centered at five cropland sites accessed through Google Maps.  
transpiration by using surface conductance and introducing mechanism-based aerodynamic conductance, which is contingent upon wind speed, surface roughness, vegetation height, and atmospheric stability [24]. In the MOD16 model, daily ET is the sum of ET from daytime and nighttime, where daytime is determined by downward solar radiation being above 0. During the daytime, stomatal conductance is assumed to be controlled by vapor pressure deficit and daily minimum air temperature. During the nighttime, the stomata are assumed to be completely closed, leading to zero plant transpiration through the stomata. However, transpiration through the leaf boundary layer and leaf cuticles still occurs. Meanwhile, ET comprises evaporation from wet and moist soil, evaporation from the rainwater intercepted by the canopy, and transpiration through stomata on plant leaves and stems. Canopy conductance for plant transpiration is calculated by using LAI from the combination of MOD15A2H and MCD15A2 HCL to scale stomatal conductance up to canopy level, which assumes that leaf area and FPAR do not vary during a given eight-day period. The Global Modeling and Assimilation Office (GMAO) meteorological forcing dataset is used to drive the MOD16 model, including average and minimum air temperature, incident PAR, and specific humidity.