Remote sensing and ground based analysis of drought in Karnali River Basin (KRB), Nepal

Abstract

Drought, a frequent and impactful climatic extreme, has significant impact on the context of topographically varied Karnali River Basin (KRB) in Nepal. This study executed a comprehensive assessment of drought dynamics by employing two key indices based on Vegetation Condition Index (VCI) derived from Moderate Resolution Imaging Spectroradiometer (MODIS) data, and single parameter based Standard Precipitation Index (SPI) from ground based station data, with a focus on spatial and temporal variations. Remote sensing and Ground based indices were utilized across all seasons within a year, while also establishing correlations between vegetation dynamics and climatic parameter (precipitation). MK-Trend test have been performed to analysis SPI trend inside the basin. Drought indices derived by climatic data were correlated with Southern Oscillation Index (SOI) to evaluate relationship with indices and seasons. This analysis revealed that drought has occurred more than half of the study period, underscoring its recurrent nature inside the KRB spatially. In particular, the winter and monsoon SPI drought patterns exhibited an alarming increasing rate, which can be surmounted by concerning decrease in precipitation by 4 mm per year. Concurrently, the VCI time series demonstrated an upward trajectory of drought occurrences during the pre-monsoon and winter seasons inside the basin emphasizing increasing winter drought in the lower altitude and summer monsoon drought in the whole basin accessing 50% area experienced vegetation failure temporally. Further investigation revealed that the manifestation of drought is intricately linked not only to local conditions but also to larger climatic oscillations, notably the SOI. Negative value examined between drought events and SOI index in the monsoon season, with enhancing El-Nino phase. The correlation between VCI and precipitation, particularly when precipitation leads VCI by one month, proved to be a robust and noteworthy observation, emphasizing the predictive utility of this relationship. Notably, during the pre-monsoon season, the correlation nearly doubled when a one-month lag in VCI-precipitation is considered. Lower elevations face heightened pre-monsoon and winter drought, while higher altitudes encountered intensified monsoon-related droughts. This results from shifting atmospheric circulation patterns, uneven convection, and increased temperatures, leading to excessive evaporation and soil moisture loss within the year. These findings hold crucial implications for water resource man-agement, agriculture, and overall resilience strategies in the face of changing climatic conditions inside the KRB. Keywords: Drought, SPI, VCI, SOI, Nepal, KRB.