Water availability and crop growth at the crop plot level in South Africa modelled from satellite imagery

Although the effect of weather on crop growth has been studied widely, the contribution of other water sources has been less well studied, mainly due to data limitation. To address this gap, the current analysis considers the importance of water availability on crop growth by taking advantage of crop field boundaries and information on South Africa's four major grain producing provinces (Northwest, Mpumalanga, Free State and Gauteng) provided by the Agricultural Geo-referenced Information System dataset. To capture crop growth along the crop growing cycle at the plot level, the MODIS's MOD13Q1 dataset of 16-day normalized difference vegetation index (NDVI) was used. To estimate the determinants of crop growth, weather effects were considered and represented by rainfall and reference evapotranspiration satellite derived data provided by the National Oceanic and Atmospheric Administration's RFE and GDAS dataset, respectively. Hydrologic and irrigation determinants were estimated based on the HYDRO1K river network dataset produced by the US Geological Survey. The results show that although weather is an important explanatory factor, other sources of water, such as irrigation, proximity to perennial and ephemeral rivers, and stream flow are also influential. Taking into account the interaction effects between weather and water availability related factors is also important to determine the effect of water availability on crop growth.