Estimating the relative performance of rainfed crops using Fraction of Absorbed Photosynthetically Active Radiation index, land, and field data

In most regions of Iran, including the province of Lorestan, the majority of agricultural activities are conducted in the open air. Climate risks have a significant impact on agricultural productivity. Drought and its effects are among the most significant natural threats to the agriculture industry in that area. The purpose of this study is to investigate the effects of drought on the yield of rain-fed crops in Aleshtar county, Lorestan province. To achieve this goal, a combination of field methods, remote sensing, and statistical methods was employed. During ground surveys, data required for laboratory operations (direct method) and measurements using AccuPAR and MODIS sensor images were collected (indirect method). In addition, precipitation data from synoptic stations in the province of Lorestan over the course of 27 years (1991-2017) were utilized to calculate the drought and its impact on yield. According to the calculated drought indices, in the province of Lorestan and the county of Aleshtar, the trend of increasing drought and the recurrence of long-term cycles of wet and drought are evident. The study of phenology characteristics of rainfed crops (barley) in relation to climate conditions revealed that an increase in thermal and water stress has a direct effect on the performance of rainfed crops. Therefore, an increase of 2.5 °C in the average temperature, combined with a lack of moisture supply during flowering, results in a decrease in the number of seeds per spike (16 seeds per spike) and, consequently, a decrease in the plant's yield. At various growth stages of rain-fed plants, the correlation index between LAI harvested by direct methods and remote sensing methods ranges between 0.57 and 0.96. This value represents the precision of remote sensing techniques. From 1991 to 2017, the correlation index values between the yield of rainfed plants, especially wheat and barley, and the values of various drought indices indicate a positive and direct relationship between yield and drought index values. The correlation index between yield and drought index values reaches its maximum value during 1-6 months, and its value decreases as time scales become longer. The physiological properties of various products are one of the primary causes of this circumstance. On the basis of the obtained results, it can be concluded that the increase in drought and heat stress in the province of Lorestan and the county of Aleshtar has caused a decrease in yield at various stages of plant growth and an increase in water demand for a variety of rainfed crops.