Assessment of agricultural drought indicators impact on soybean crop yield: A case study in Iowa, USA

Agricultural drought is a condition of insufficient soil moisture caused by a deficit in precipitation over some time period. Soil moisture drops to a certain extent, adverse to the crop yield, and then reduces the production of crops. Soybean is one of the most important sources of oil and protein in the world. It is vulnerable to recurrent drought condition in the U.S. state of Iowa. This study was conducted to identify agricultural drought indicator that strongly correlated with soybean crop yield. Detail crop data (e.g., soybean crop yield, etc.) were collected from the USDA's National Agricultural Statistics Service (NASS). A region of interests is defined based on the MODIS 16-days 250m resolution vegetation index synthetic products (MOD13Q1) and daily land surface Temperature/Emissivity 1km resolution products (MOD11A1) from 2000 to 2013 in Iowa, which were used to compare three kinds of remote sensing derived agricultural drought monitoring indicator of crop water demand status: (i) Crop morphological indices (e.g., NDVI/VCI); (ii) Crop physiological indices (canopy temperature, e.g., TCI); and (iii) Crop comprehensive indices (e.g., VSWI). Drought cumulative effects were considered according to the specific soybean crop growth stages including from planted to emerged, vegetative period (from emerged to blooming), reproductive period (from blooming to setting pods), and growing season (from emerged to dropping leaves). The impacts of drought duration on the soybean crop yield by both of indices were analyzed. These results imply that physiological indices and comprehensive index were more correlated to assess the effect of drought on soybean yield. Drought indices accumulated in reproductive period (from blooming to setting pods) are highly superior to other accumulated for impacting on soybean yield, while over the growth season (from emerged to dropping leaves) is also highly correlated with the total yield assessment. The results can aid on evaluating the effects of drought on soybean yield in different growth stage. This could be very useful to providing auxiliary decision-making information for drought relief, agricultural manager and grain merchant to plan solutions and prepare for potential drought in advance.