## 2 Materials and Methods  
### Study Area  
The study area Texas (area = 695,622 km\({}^{2}\)) has ten climate zones and 12 ecological regions [61; 62]. The ten climate zones are: High Plains (HP), Low Rolling Plains (LRP), East Texas (ET), Edwards Plateau (EP), South Central (SC), Trans Pecos (TP), Upper Coast (UC), North Central (NC), Southern (S), and Lower Valley (LV). The twelve ecoregions are: Arizona/New Mexico Mountains (ANM), Chihuahuan Deserts (CD), High Plains (HP), Southwestern Tablelands (SWT), Central Great Plains (CGP), Cross Timbers (CT), Texas Blackland Prairies (TBP), Edwards Plateau (EP), East Central Texas Plains (ECTP), South Central Plains (SCP), Southern Texas Plains (STP), and Western Gulf Coastal Plain (WGCP) (Figure 1). All of the climate zones have multiple ecoregions, which show significant diversity in landscapes, topography, and land-use in the state. ANM ecoregion is very small in area and can be included in CD ecoregion. CD ecoregion has desert valleys, plateaus, and wooded mountains, and receives the lowest precipitation in the state. HP ecoregion, a relatively high-level plateau, is mostly covered with cropland. SWT and CGP ecoregions, characterized as a rolling plain, are mostly covered with prairie grasslands. CT and TBP ecoregions are located in north-central Texas. CT is covered with high-density trees and regular plains and prariies, whereas TBP is covered with tall grass and prariies. EP ecoregion, known as the Texas Hill Country, has many springs, stony hills, and steep canyons. SCPlocated in south-west Texas, is covered with thorny shrubs and trees and scattered patches of palms and subtropical woodlands, whereas WGCP, located along the coast of Gulf of Mexico, is covered with cropland, tall prariies, and marshland. SCP and ECTP ecoregions are dominated by forestland and pastureland, respectively. SCP ecoregion is covered with pines and oaks and characterizes the forest of the East Texas Pinewoods, whereas ECTP ecoregion is covered with strips of prairie grassland and can be described as oak savannah mixed with grassland [63]. Texas is dominated by deserts, pine forests, shrubland, and cropland. It shares the Rio Grande River with Mexico, and its size and physical geography influence its diverse climate. Three geographical features, which significantly influence the climate of Texas, are the Rocky Mountains, Central and Eastern parts of the continent of North America, and the Gulf of Mexico. The Rocky Mountains influence Texas climate by acting as a barrier to air traveling from east to west or west to east. While the Central and Eastern parts of the continent of North America act as a pathway for airflow into Texas, the Gulf of Mexico, a source of moisture, helps to moderate Texas temperature [64]. Ten research sites (ten SMAP grid cells) from seven climate zones, which include five major land uses, were selected in this study. We selected only one SMAP grid cell from each land-use because the size of one grid cell is 9 km by 9 km, which is large enough to cover homogenous agricultural field or landscape of selected land-use type. The selected climate zones are HP, ET, EP, SC, TP, UC, and NC (Figure 1). The Upper Coast climate zone receives the highest amount of precipitation, whereas the Trans Pecos receives the lowest amount of precipitation in Texas (Figure 2). In general, the climate zones of the eastern and southern parts of the state receive higher rainfall compared to the climate zones located in the north and west. Texas' climate varies widely across the state, from humid and hot in the east and south to arid and cool in the west and north (Figure 2). The temperature gradually increases from south-east to north-west across the state. Forests are the predominant land-use type in the eastern parts, whereas shrublands are the predominant land-use type in the western parts of the state. However, most southern and northern regions of the state are dominated by cropland.