# Regional Crop Gross Primary Productivity and Yield Estimation Using Fused Landsat-MODIS Data
## 2 Materials and Methods  
### Study Area  
Montana (MT, 44\({}^{\circ}\)\(\sim\)49\({}^{\circ}\)N, 104\({}^{\circ}\)W\(\sim\)116\({}^{\circ}\)W) encompasses 380,832 km\({}^{2}\) of the northwest continental United States and is the largest landlocked state in the country. Agriculture is the largest economic sector in MT [44]. The Cropland Data Layer (CDL) from National Agricultural Statistics Service (NASS) provides cropland classification maps over the continental U.S. domain from 2008 to the present; the CDL is a geo-referenced and crop-specific land cover data layer derived using relatively fine-scale satellite imagery (e.g., Landsat 4/5/7/8, Terra MODIS NDVI, etc.) with a decision tree classifier. Croplands encompassed 29.60 \(\pm\) 1.98 percent of the entire MT land area over the eight-year (2008 to 2015) study period based on the 30-m CDL product. The MT croplands are dominated by seven major crop types, including maize, barley, durum wheat, spring wheat, winter wheat, alfalfa and peas, which were selected in this study for subsequent GPP and crop yield predictions. The MT croplands are mainly distributed in the northcentral and northeastern portions of the state and are more dispersed in central and southern areas for the 2008 to 2015 study period (Figure 1a). Alfalfa is dispersed across MT, but is concentrated in the northwest and central areas of the state. Barley is also grown in most counties, but is more prevalent in the north central portion of MT, together with spring wheat and winter wheat. Some spring wheat is also located in the northeast, where durum wheat is also planted. Peas are planted near barley, spring wheat, winter wheat and durum wheat. Maize is mainly distributed in the southeastern, northeastern and south central portions of the state over the 2008 to 2015 record. Crop types and planted areas show strong annual variations in MT from 2008 to 2015 (Figure 1b). For example, the total cropland area of the seven major MT crops ranged from an annual minimum of 4.13 \(\times\) 10\({}^{6}\) hectares in 2009 to a maximum of 6.34 \(\times\) 10\({}^{6}\) hectares in 2014; whereas individual crop types ranged from 0.7% (maize) to 30.7% (spring wheat) of the total planted area in MT over the study period.  
Figure 1: (a) Cropland distributions for the seven major crop types in Montana in 2015 based on the 30-m Cropland Data Layer from National Agricultural Statistics Service(NASS), including alfalfa, barley, maize, durum wheat, spring wheat, winter wheat and peas; areas in white denote other crop types and land cover areas; (b) planted areas for the different crop types per year from 2008 to 2015 are represented as a proportion (%) of the total planted area for all seven MT crop types.  
MT is located in the northern temperate latitudes and has a semi-arid continental climate with an extended winter frozen season, where the potential growing season extends from the end of snow cover depletion in spring to the onset of persistent frozen temperatures in autumn. Based on the reported planting and harvest dates for field crops ([PERSON] et al., 2015), the planting dates for the major MT crop types (except winter wheat) range from early April to mid-June, whereas harvest dates extend from late July to early October. Winter wheat is usually planted in early September and harvested during the following July to August period. Thus, for this study, we assume an annual period for seasonal crop development, harvest and yield from April to September over the 2008 to 2015 record.