Monitoring crop water use using unmanned aerial vehicle (UAV) and surface energy balance algorithms: a case study of Vaalharts Irrigation Scheme, Northern Cape Province, South Africa

Abstract

Crop water use is an important component of the hydrological system. The fact that there is continuous and persistent drought in the face of global environmental change and ongoing tensions over water use is enough to warrant crop water use monitoring. This study applied different methods to quantify crop water use in an 18-hectare barley field accurately. Soil water content spatial distribution was determined by integrating ground measurements using a Daiki-311A soil moisture meter probe, an HS2 Hydrosense II handheld sensor and by calibration of soil electrical conductivity fluxes recorded using an electromagnetic induction instrument (EM 38). Estimating evapotranspiration was done using different approaches, including smart weather station data for reference evapotranspiration by applying Penman Monteith, Priestley and Taylor’s methods. In contrast, actual evapotranspiration was estimated using two smart field weighing lysimeters and Unmanned Aerial Vehicle (UAV) data in a temperature-vegetation triangular algorithm. The results for this research are presented as graphs and spatial distribution maps, while their comparisons and evaluations were made using statistical approaches. The findings were that the determination of crop water use and soil water content makes it possible for stakeholders in agriculture to determine irrigation requirement, irrigation scheduling, water use monitoring, and irrigation planning, which is positively attributed to water resources management. The evapotranspiration obtained from the UAV remote sensing simulation showed coherence with the lysimetric evapotranspiration. The soil water content obtained using ground handheld sensors was consistent with the soil water content derived from the AUV data set as a moisture index. Since unmanned aerial vehicles, lysimeters, and electromagnetic instruments cannot be used over large areas, mapping crop water use under the surveillance of high-resolution sensors integrated with remote sensing techniques will be improved in the future. The accurate and timely monitoring of crop water use will improve agricultural stakeholders' decision-making towards irrigation scheduling and water management while optimising yields