**4. Discussion**

We saw that the escalated variation in hydro-meteorological parameters revealed by the previously discussed CV values was reaffirmed by the MK-S test for the time period of 2000–2016 compared to the years 1956–1999. The trend of rainfall pattern in the LRB during the two time spans was shown to have undergone a decrease in the latter time span that was also of greater magnitude, as exposed by Sen's slope estimator. However, the decrease was not significant. Looking at the temperature change trend in the LRB during the two time spans, a significant increase in the temperature was revealed by the MK-S test, with a rapid rise in temperature in the study time span compared to the previous long-term time period. The authors of [81] also reported a significant increase in all the QTP basins including the LRB, with a range of 0.03–0.06 ◦C year<sup>−</sup>1, which was similar to the findings of our study. The LR discharge trend revealed a non-significant increase in the former long-term time span. Conversely, for the study time span, we saw a significant decrease in the LR discharge with a larger magnitude, as estimated by the Sen's slope. We saw that the rainfall also decreased along the same time span, and the temperature was seen to have risen, which could have a serious impact on the LR discharge in the form of lower precipitation and aggravated evapotranspiration in the LRB. However, it can be seen that the decrease in the magnitude of LR discharge during 2000–2016 was far more, which clearly leads to the conclusion that in addition to the climatic phenomena, some other factors were causing the pronounced decrease in LR discharge during 2000–2016. During these years, major hydraulic interventions were witnessed in the LRB in the form of dam construction, including the Zhikong hydropower project (2006) and the Pangduo hydropower project (2013). These hydraulic structures impounded the water and caused the prominent decrease in LR discharge.

The MK-S test revealed a decreasing LR trend for all the seasonal flows, and the decrease reached significance in wet summer season and the dry winter season. Sen's slope estimator revealed that the greatest reduction in the LR discharge was experienced in the wet summer season, followed by the dry winter season. The spring season was shown to experience the lowest reduction in discharge. This phenomena of the LR discharge trend could be explained by the fact that the wet summer season was the peak flow time in the LRB, where the major portion of LR discharge was generated (~90%) during these months. This water was stored in the reservoirs built on the LR for the succeeding dry wet season with a minimum rainfall and was responsible for the significantly decreased LR discharge during peak flow season. The higher variability and non-significant decrease in the LR discharge during spring season could be attributed to the snow and glacier melting, which is a completely-climate driven phenomena in the LRB. Thus, the variation of LR discharge during spring season is highly prone to the snowfall received during the winter season. In LRB, from April to May is a melting season when air temperature is, on average, above zero [81]. This snowmelt contributes to LR discharge and hence stabilizes

the effect of rising temperature in the form of evapotranspiration and may be a reason for smaller decrease in LR discharge during spring season. Increasing air temperatures lead to less snow accumulation in the winter and an earlier peak runoff in the spring, as well as reduced flows in summer and autumn [82–85]. The change in the streamflow regime results in a substantial impact on regional water resources and seasonal water supplies [86]. For the dry winter LR discharge, a significant decrease with a greater magnitude and lower variability compared to the spring season could have been a possible manifestation of the increase rate of temperature. The MK-S test showed a significant and intensified increase in temperature of the LRB for the years 2000–2016. Similar behavior was reported in [25] for the LRB, where the increase rate of the minimum temperature was found to be higher in spring and winter than in summer, whereas the maximum temperature showed the opposite trend. This increased minimum temperature is causing an overall warming of the LRB and is showing its effects in the form of evapotranspiration with minimal rainfall to balance it, thus leading to a decreased LR discharge during the dry winter season. Additionally, the water demand in the LRB during the dry winter months is met by ground water abstraction and the reservoir-stored water during the wet summer season. This is again a factor that affects the LRB's hydrological behavior in dry spells of a year.

While predicting the LRB streamflow, we saw that in both situations, the LR discharge was predicted to decrease under the reservoir influence. This could be attributed to the inertial characteristic of the ARIMA model forecasts. If the historical data rise rapidly right before the peak value, they cannot be foreseen by the ARIMA model and the peak value would therefore be underestimated; however, if the rise is slow and steady, the rising trend would be expected to continue after the peak by the ARIMA model [28]. Here in the case of observed LR discharge, we saw a decreasing development through the years 2000–2016, which was confirmed by the MK-S test results presented earlier. Thus, ARIMA predicted an obvious decrease in the observation-based forecasted LR discharge. For the SWATsimulation based LR discharge, the ARIMA model again predicted a decreasing ye<sup>t</sup> stable future streamflow for 2017–2025, following the same behavior as the SWAT-simulated flow (Figure 7b).

The current study was intended to overcome the data scarcity issue of the study area, which is a major concern of the QTP catchments. In the current study, data availability on reservoir operations was a major constraint. Additionally, the data quality and availability for hydro-meteorological parameters included in the study were a prime concern for the trans-boundary Lhasa River.
