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Understanding the impact of drought on agriculture is critical for reducing drought-related yield losses in the Koshi River Basin, Nepal. This study used the Standardized Potential Evapotranspiration Index (SPEI) to analyze drought during the crop growing season. The Lagrange interpolation method was employed to identify expected crop yields and to quantify crop yield losses in the Koshi River Basin, Nepal. Finally, the correlation method was applied to observe the relationship between yield loss and drought in the basin. Spatially, the growing seasons of maize, rice, and wheat in the hill region exhibited a more pronounced increase in the drought trend compared to the mountain and Terai regions. Similarly, the maize growing season demonstrated a more significant increasing drought trend than rice and wheat across all regions. However, the wheat growing season in the mountain exhibited an increasing drought trend. The correlations between the detrended SPEI and crop yield loss revealed that maize yields in the Terai and mountain regions are sensitive to drought in both the early stages (from sowing to tasseling) and later stages (milking, seed ripening) in the hill. Similarly, rice yields are sensitive to drought from the plant elongation to ripening in both the hill and Terai regions. Wheat yield analysis showed a weak positive relationship in all regions. The findings of our study can enhance understanding of drought conditions and their impact on crop growth stages, thereby assisting local farmers and agricultural institutions in managing crop yield loss. [ABSTRACT FROM AUTHOR]

The quantitative assessment of crop yield loss due to drought is critical for the development of the agricultural sector to improve the productivity. This study aimed to estimate and analyze the spatiotemporal patterns of crop yield loss in response to drought using the Lagrange interpolation method, wavelet analysis, and sequential Mann–Kendall test in Nepal's Koshi River Basin, covering the mountain, hill, and Terai (low-land) regions from 1987 to 2016. The findings indicate that average crop yield loss was common after 2000, with the Terai, hill, and mountain regions experiencing the greatest loss in maize, rice, and wheat crops, respectively. The average annual rice and wheat yield losses rate were highest in the mountains, while maize yield losses were highest in the Terai. The mountain region experienced an abrupt change in wheat yield loss, showing a significant increasing trend. The hill region showed a significant increase in maize and wheat yield loss, while a decrement in rice yield loss was observed. The periodic variations of maize, rice, and wheat between 1987 and 2016 revealed significant yield loss after 2000. The characteristics of the first and second key periods for crop yield loss demonstrated a variation period, predicting that crop yield loss would either enter a high yield loss or low yield loss period shortly after 2016. The study findings provide a detailed assessment of crop yield loss at the river basin level, highlighting the urgent need to develop a crop yield loss mitigation plan in the agricultural sector. [ABSTRACT FROM AUTHOR]