**2. Materials and Methods**

#### *2.1. Study Area*

Pakistan lies between latitude 23.5◦–37.5◦N and longitude 62◦–75◦E in the western region of South-Asia with an area of 803,940 km<sup>2</sup> (Figure 1a). Pakistan is bounded by China at its north, Iran and Afghanistan at the west, India at the east, and the Arabian Sea at the south. The study area has complex topography with a diverse climate. Elevation of Pakistan ranges from 8600 m (at Hindukush-Himalaya mountain ranges) at the extreme north to 0 m (at the coast of Arabian Sea) [47]. The climate of Pakistan changes abruptly, ranging from glacial to humid, arid and hyper-arid regions. According to climate variability, the study area is divided into glacial, humid, arid and hyper-arid climate regions (Figure 1b).

**Figure 1.** Study area and geographical location of rain gauges (RGs). (**a**) The elevation map of Pakistan derived from Shuttle Radar Topography Model (SRTM), (**b**) The four climate regions considered in this study.

The glacial region, situated in the extreme north of Pakistan, is characterized by permanent glaciers and snow cover. Mean elevation of the glacial region is 4158 m, with the mean annual precipitation of 348 mm. Snow and glaciers from the glacial region melt in summer and feed the Indus river and its branches. Water from snowmelt is the primary source of agricultural, domestic, and industrial water use in Pakistan. Moreover, the country is threatened by excessive snow and glacier melt that causes acute flooding, such as the 2010 flood which severely damaged the infrastructure, the country's economy and took thousands of lives. The glacial region is located between 34◦N to 38◦N and comprised of the world-famous mountains of Hindukush Himalayas, which are famous for snow after the polar regions.

The humid region consists of mountains ranges of Hindukush–Karakoram–Himalaya (HKH) ranges, and all of the main rivers originating from these mountains, including Indus, Kabul, Gilgit, Hunza, Swat, Panjkora, Kurram, and Jhelum rivers. Mean elevation and mean annual precipitation of the humid region is 1286 m and 852 mm, respectively. The humid region is the hydraulically developed region of the country which consists of the largest dam, Tarebla dam, and Mangla dam. The Tarbela dam, constructed on the Indus river, has 3500 MW of capacity, while the Mangla dam constructed on Jhelum river has 1000 MW hydropower capacity [48]. Moreover, the humid region is built up with extensive barrages, headworks, and a developed integrated irrigation canal system. The secondary purpose of the hydraulic structures is to support the agriculture sector of the country by meeting the extensive irrigation water requirements [48].

The arid region is mostly comprised of the major agricultural regions of Punjab province. The Indus river and other rivers flowing through this region are the primary sources of water for agricultural, domestic, and industrial purposes. Mean elevation of the arid region is 663 m with mean annual precipitation of 322 mm. Extreme west (elevated mountainous areas) of this region is cold in winter

(with snowfall in December and January) and hot in the summer seasons. The remaining of the arid zone is dry and hot in nature where maximum temperatures are recorded in Sibbi and Jacobabad. The area is plain with some area included in the famous Thar Desert [49].

Sindh and Balochistan provinces and the south part of the Punjab province of Pakistan lie in the hyper-arid region. Most of the hyper-arid region is comprised of deserts, plateaus, barren lands, dry mountains, and coastal region along the Arabian Sea. Mean elevation and mean annual precipitation in the hyper-arid region are 444 m and 133 mm, respectively.

Precipitation in Pakistan is spatially varying from the maximum of 1500 mm/a in the north to a minimum of 100 mm/a in the south. Monsoon and winter (western disturbance) are the dominant seasons where maximum rainfall occurs [50]. Monsoon (July to September) precipitation in Pakistan commences from the Bay of Bengal entering Pakistan from northeast and east sides. Heavy and intense precipitation accounting 55%–60% of precipitation per annum occurs in Pakistan during the monsoon season [51]. Winter (December to March) precipitation originating from the Mediterranean Sea enters Pakistan through Afghanistan and Iran. Only 30% of total precipitation (moderate magnitude) occurs during winter. Precipitation across Pakistan varies spatially in magnitude; low precipitation in the glacial region from 34 to 36◦ N (<100 mm/month), high in the humid region from 29 to 33◦ N (>700 mm/month in North-East) and low again in the hyper-arid region from 24 to 28◦ N (around 100 mm) [47].
