Spatiotemporal variations of net primary production using remote sensing and field data

Net primary production (NPP) is commonly used to measure the productivity of steppe grasses and has been extensively studied. However, NPP variations at a watershed scale as influenced by precipitation and environment factors (e.g., soil properties) are not well documented in the existing literature. This chapter examines spatiotemporal variations of NPP across a typical steppe watershed using remote sensing and field data and determines the effects of precipitation on the NPP variations. Besides the Moderate Resolution Imaging Spectroradiation (MODIS) data from 2000 to 2010, field data on NPP, soils, and groundwater were collected and analyzed by using advanced statistical methods (e.g., Mann-Kendall test and correlation) and visualization techniques. The results indicate that the NPP for a downstream area within the study watershed tended to be smaller than that for an upstream area. The NPP fluctuated with precipitation synchronically and was largely affected by the May-to-July precipitation for a given year. The May-to-July precipitation was attributable for 12% to 65% of the NPP variances and played a more prominent role in the growth of steppe grasses in a lower area than an upper area. The NPP spatial pattern and such spatially differentiated precipitation effects depended on topography, soil texture, soil permeability, soil salinity, and livestock grazing intensity.