What Caused the Winter Drought in Western Nepal during Recent Years?*,+

Abstract

Western Nepal has experienced consecutive and worsening winter drought conditions since 2000, culminating in a severe drought episode during 2008/09. In this study, the meteorological conditions and a historical perspective of the winter droughts in western Nepal were analyzed using instrumental records, satellite observations, and climate model simulations. Meteorological diagnosis using atmospheric reanalysis revealed that 1) winter drought in western Nepal is linked to the Arctic Oscillation and its decadal variability, which initiates a tropospheric short-wave train across Eurasia and South Asia; and that 2) the persistent warming of the Indian Ocean likely contributes to the suppression of rainfall through enhanced local Hadley circulation. Simulations from the phase 5 of the Coupled Model Intercomparison Project (CMIP5) sets of historical single-forcing experiments indicated that the increased loading of anthropogenic aerosols is also a compounding factor in the precipitation decline during the later decades. It is therefore conceivable that the recent spells of decadal drought in Nepal are symptomatic of both natural variability and anthropogenic influences. Given the observations that winter precipitation has declined to near zero while groundwater has hardly been replenished, appropriate management of western Nepal's water resources is both critical and necessary.

Rising Precipitation Extremes across Nepal

“…Nevertheless, increasing CDD observed over the study area is consistent with the most widespread and worst drought observed in recent decades across the country [99,101,110,111]. Rise in CDD clearly indicates the prolongation of the dry period across the country, implying certain changes in the seasonality of prevailing precipitation regimes [95,96].…”

“…Consistently, weakening influence of the western disturbances over the central Himalayas has also been found [102], and in line with this, decreasing winter precipitation has been reported in the adjoining western Himalayan region in India [103]. Further, Wang et al [101] have attributed this decline to three main factors: (1) decadal trend towards negative phase of arctic oscillation in recent decades that has created a local mass flux circulation with descending branch over western Nepal; (2) the Indian ocean warming, and; (3) the anthropogenic aerosol loading. It is pertinent to mention that the winter precipitation, though low in volume, plays an important role in meeting the water demand of the winter crops and in feeding the rivers through accumulating their headwaters with snow that melts during the dry pre-monsoon season [42].…”

“…Khatiwada et al [98] have also reported a decreasing winter precipitation over the Karnali basin in western Nepal for the 1981-2012 period. Based on GPCP and satellite-based datasets, Wang et al [101] have likewise identified declining winter precipitation over the western region of Nepal in recent decades. Consistently, weakening influence of the western disturbances over the central Himalayas has also been found [102], and in line with this, decreasing winter precipitation has been reported in the adjoining western Himalayan region in India [103].…”

“…It is pertinent to mention that the winter precipitation, though low in volume, plays an important role in meeting the water demand of the winter crops and in feeding the rivers through accumulating their headwaters with snow that melts during the dry pre-monsoon season [42]. Particularly for the western hills and mountainous regions where food production is largely dependent upon rain-fed agriculture, decreasing winter precipitation may affect the winter crop production [101] of wheat, barley and potatoes, a major crop of the hills and mountains. Moreover, the decreasing winter precipitation in the region, where winter precipitation is substantially higher than in other regions, could also lead to a reduction in pre-monsoon season river flows, which are largely dependent on the snow and glacier melt during the dry season.…”

Understanding forest fire patterns and risk in Nepal using remote sensing, geographic information system and historical fire data

“…In the eastern parts of the Terai, March is the peak fire month while in western Terai and most of the Siwalik hills, fire incidences peak in April. We observed a higher number of forest fire incidences in the year 2009, which corresponds with the driest year of the study period experiencing 50% less rainfall than average during the winter season, followed by the late start of the monsoon period for the year (Wang et al 2013). The annual length of the fire season was calculated using the first and last fire incidence that occurred for individual districts.…”