Climate change is a global concern. Global temperature is rising and it is projected that the global mean temperature will rise between 1.4 and 5.8 °C by the year 2100 [1
]. The rising temperature affects all major components of hydrological systems: it induces change in precipitation pattern, enhances melting of snow and ice, and increases evaporation, altering soil moisture and runoff [3
]. The relationship between the climate system and the hydrologic cycle underlies the risk of floods and drought and possible future influences of global warming on water resources [5
Mountainous environments are considered sensitive to climate change [6
]. In spite of the fact that the climate of the Himalayan region is changing rapidly [7
], the majority of precipitation studies of South Asia have excluded the Himalayan belt due to its complex topography and lack of rain-gauge coverage [8
]. Climate models have less skill in capturing precipitation patterns in high mountains [10
]. There is very low consistency among climate models in precipitation predictions in Himalaya region for either the winter or the monsoon seasons; an overall increase in annual precipitation is projected, but the magnitude of change is low [5
]. The Himalayas are at the headwaters for the major river systems of Asia [12
], and understanding how climate change is operating in the region is critically important [14
]. Change in precipitation is expected to significantly affect cryospheric processes and the hydrology of headwater catchments in the Himalayas [15
]. Ice core analysis showed that South Asian monsoon variability in the Himalayas has been significant [16
Climate of Nepal is dominated by the southeasterly monsoon coming from the Bay of Bengal. This monsoon provides most (around 80%) of the annual precipitation during the summer months from June to September [17
]. The summer monsoon is more active in eastern and central Nepal while a winter monsoon is active in western Nepal [18
] particularly on the leeward side of the High Mountain and High Himalaya in the northwest [8
]. The northern side of the Himalayas receives scanty precipitation year-round and therefore is known as a ‘rain shadow region’ [20
]. Due to the steep slopes and rugged topography, precipitation estimation in Nepal is very difficult. The topography of Nepal varies from lowland plain in the south to the high Himalaya range peaks (more than 8000 m) towards the north, which has a profound influence on precipitation. Climatic regimes similarly vary from subtropical in the southern plains to the alpine in the high mountains [18
]. Large variation of precipitation occurs even over small distances due to highly undulating surface topography with high mountains and deep valleys [21
]. Analysis of observed precipitation data in Nepal is very limited and one of the reasons for this is the relatively short period of data availability [17
]. Previous analyses of precipitation data have not revealed any significant trend [13
In this paper, we analyze the spatial and temporal trend of precipitation across different agro-ecological zones of the Gandaki River basin (GRB). Due to topographic variation and orographic effects, precipitation estimation is a complex task and the analysis combining data from different altitudes to report a single pattern cannot distinguish variations in trends between agro-ecological zones. Therefore, it is important to understand the distribution of precipitation and its trend in different altitudinal belts separately. The GRB includes all the agro-ecological zones of Nepal, from plains (Terai) in the south to the high Mountain and Trans-Himalaya in the north. Additionally, the Trans and the high Himalaya regions of the basin receive the lowest amount of the precipitation and the Pokhara area in the Hill region receives the highest precipitation in the country. Thus, we focus on the GRB as a microcosm of the sharp precipitation variations and disparate trends found in the topographically and culturally complex Himalayas region.
The GRB receives 78% of annual precipitation in the summer monsoon season, and the contribution of monsoon precipitation was the highest in Hill and Mountain and lowest in Trans-Himalaya regions. In general, Nepal receives 80% of the total annual precipitation in monsoon season [8
]. The lower proportion of monsoon precipitation in the Trans-Himalaya region is due to the fact that this region lies in the leeward side of the Himalayan Massif. This region receives winter precipitation under the influence of western disturbances. The Gandaki River Basin receives the highest amount of precipitation in July followed by August for all the regions except the Trans-Himalaya, which shows peaks in May and December.
The comparison of total annual precipitation across agro-ecological zones showed that the Hill region receives the highest annual precipitation and the Trans-Himalaya receives the lowest. The same pattern applied for monsoon precipitation. The Pokhara and Lumle areas that receive the highest amount of precipitation in the country [38
] lie in the Hill region of this basin. These areas are on the windward side of the Annapurna-Machhapuchhre mountain range. On the contrary, Manang and Mustang districts, which receive the lowest amount of precipitation in the country [39
], lie in the leeward side of these mountains in the Trans-Himalaya region of the GRB. In high mountain areas, the local topography also plays an important role [40
] in precipitation amount. One additional contributor for the low amount of precipitation measured in the Trans-Himalaya region may be the under-estimation of snowfall by both surface rain gauges and remote sensing [10
The coefficient of variation in the amount of precipitation averaged for the whole GRB basin was highest for winter season, followed by the post-monsoon. The amount of precipitation in the monsoon season was the most stable. When we compare variability across agro-ecological zones and seasons, Trans-Himalaya had the greatest variability in the post- monsoon season followed in Mountain in the winter season. These patterns are related to the sporadic nature of the precipitation patterns in these seasons, which are dominated by the precipitation associated with the tropical disturbances during post-monsoon season and by western disturbances in the winter season. During winter season, occasional strong northeasterly winds from the Siberian High bring precipitation or snow across all agro-ecological zones, but particularly high-altitude areas [41
The analysis of precipitation trends for different seasons across different agro-ecological zones revealed that the monsoon precipitation is increasing whereas precipitation in other seasons is decreasing, although most of these trends for 1981–2012 are not statistically significant (Table 3
). These trends are consistent with previous studies [9
] that report spatial variations in the precipitation in the Nepal and in the Himalayas. It is interesting to note that the precipitation records over the monsoon region around the globe reveal a decreasing trend [43
], unlike what is seen in Nepal. In this study, the annual precipitation showed a large decreasing trend of 6.3 mm per year in the Mountain region though the trend was not significant. This is also supported by the findings of Shrestha et al.
] for the entire Nepal. The trends for annual precipitation indicate that the dry parts of the basin (Trans-Himalaya and Mountain) could become even dryer as there is decreasing trend and the wet parts (Hill region) could become even wetter as they show an increasing trend. Both of these patterns can affect the livelihood of the people in the region.
The precipitation analysis shows that the recent decades (1990s and 2000s) are dryer than earlier decades. Decadal anomalies in precipitation within the basin range from −60% to +90%. The 1970s followed by 1980s are the wettest decades in terms of annual precipitation for the Gandaki River Basin as a whole. Increased amount of precipitation in monsoon may increase the severity of water-induced disasters, such as landslides in the Hill region and floods in the lowland region of the basin. An increased frequency and intensity of extreme precipitation events may cause flash floods [44
]. In the Hill region, the annual precipitation rate is increasing but the number of rainy days does not show any trend, indicating more heavy precipitation events. Fragile and sensitive geology, deforestation and the heavy precipitation make the Hill region very susceptible for soil erosion and landslides [45
]. The decreasing post-monsoon precipitation may adversely affect paddy rice production as this time coincides with the sensitive stage of the paddy (season for spikelet formation, fruiting and ripening) that demands more water [48
]. In addition, the decreasing winter precipitation may lead to failure of winter crops such as wheat, barley and potatoes.
The monsoon onset (arrival date) does not show any significant trend but the withdrawal (departure date) is delaying, making the duration of summer monsoon period longer. An earlier study for Nepal found that the number of rainy days is decreasing but the total amount of annual precipitation does not show any trend [49
]. This can be attributed to intense precipitation in rainy days accompanied by prolonged drought. Although the harvesting and planting times of crops vary across agro-ecological zones, the farming system is strongly dependent on the timely arrival of monsoon rain and its distribution [50
]. Rising temperatures in combination with increasing precipitation anomalies can influence soil moisture, ultimately affecting the crop production and livelihood of the people depending upon rain-fed agriculture.
It is important to understand spatial and temporal patterns of precipitation from a hydro-climatological perspective for regional water resource management [51
] and also for socioeconomic reasons [38
]. Due to the higher number of rain gauge stations compared to temperature measurements, precipitation trends could be particularly useful to assess climatic changes in the higher Himalayas [52
]. This study could be a first step for planning and development of water resources in the GRB and could be a model to study precipitation patterns in the basins with diverse topography [53