The control of climate sensitivity on variability and change of summer runoff from two glacierised Himalayan catchments

The response of catchment runoff to climate forcing is determined by its climate sensitivity. We investigate the sensitivity of summer runoff to precipitation and temperature changes in winter-snow dominated Chandra (western Himalaya), and summer-rain dominated upper Dudhkoshi (eastern Himalaya) catchments in order to understand the nature of climate-change impact on the mean summer runoff and its variability. The runoff over the period 1980–2018 is simulated with a semi-distribute hydrologic model, which is calibrated using available discharge and glacier mass loss data. An analysis of the interannual variability of the simulated summer runoff reveals that the runoff from the glacierised parts of the catchments is sensitive to temperature changes, but is insensitive to precipitation changes. The behaviour of the summer runoff from the non-glacierised parts is exactly opposite. Such precipitation-independent runoff from the glacierised parts stabilises the catchment runoff against precipitation variability to some degree. With shrinking glacier cover over the coming decades, the summer runoff from the two catchments is expected become more sensitive to the precipitation forcing and less sensitive to the temperature forcing. Because of these competing effects, the impact of the glacier loss on the interannual variability of summer runoff may not be significant. However, the characteristic ‘peak water’ in the long-term mean summer runoff, which is caused by the excess meltwater released by the shrinking ice reserve, may lead to a detectable signal over the background interannual variability of runoff in these two catchments.

Is Climate or Direct Human Influence Responsible for Discharge Decrease in the Tunisian Merguellil Basin?

Climate change and direct anthropogenic impact are recognized as two major factors affecting catchment runoff. This study investigated the separate effect of each of these factors for runoff from the important Tunisian Merguellil catchment. For this purpose, more than forty years of hydrological data were used. The methodology was based on hydrological characterization, NDVI index to monitor land use dynamics, and the Budyko approach to specify origin of change. The results show that hydrological change is much more important upstream than downstream. The last three decades display a 40% reduction in runoff. This is associated with the direct influence of humans, who are responsible for about 78% of the variation in flow. It appears that climate change contributes to less than about 22%. The combination of increased cultivated land and decreased annual rainfall is the main reason for reduced catchment runoff. Consequently, these effects threaten the sustainable runoff, water in reservoirs, and future water supply in general. Ultimately, the available runoff remains an important parameter and a key indicator to guide the choices of decision-makers and practitioners in current and future climatic conditions. This contributes to supporting sustainable management of remaining water resources.

Unusual catchment runoff in a high alpine karst environment influenced by a complex geological setting (Northern Calcareous Alps, Tyrol, Austria)

Garber Schlag (Q-GS) is one of the major springs of the Karwendel Mountains, Tyrol, Austria. This spring has a unique runoff pattern that is mainly controlled by the tectonic setting. The main aquifer is a moderately karstified and jointed limestone of the Wetterstein Formation that is underlain by nonkarstified limestone of the Reifling Formation, which acts as an aquitard. The aquifer and aquitard of the catchment of spring Q-GS form a large anticline that is bound by a major fault (aquitard) to the north. Discharge of this spring shows strong seasonal variations with three recharge origins, based on δ18O and electrical conductivity values. A clear seasonal trend is observed, caused by the continuously changing portions of water derived from snowmelt, rainfall and groundwater. At the onset of the snowmelt period in May, the discharge is composed mainly of groundwater. During the maximum snowmelt period, the water is dominantly composed of water derived from snowmelt and subordinately from rainfall. During July and August, water derived from snowmelt continuously decreases and water derived from rainfall increases. During September and October, the water released at the spring is mainly derived from groundwater and subordinately from rainfall. The distinct discharge plateau from August to December and the following recession until March is likely related to the large regional groundwater body in the fissured and moderately karstified aquifer of the Wetterstein Formation and the tectonic structures (anticline, major fault). Only a small portion of the water released at spring Q-GS is derived from permafrost.

From the 1990s climate change has decreased cool season catchment precipitation reducing river heights in Australia’s southern Murray-Darling Basin

The Murray-Darling Basin (MDB) is Australia’s major agricultural region. The southern MDB receives most of its annual catchment runoff during the cool season (April–September). Focusing on the Murrumbidgee River measurements at Wagga Wagga and further downstream at Hay, cool season river heights are available year to year. The 27-year period April–September Hay and Wagga Wagga river heights exhibit decreases between 1965 and 1991 and 1992–2018 not matched by declining April-September catchment rainfall. However, permutation tests of means and variances of late autumn (April–May) dam catchment precipitation and net inflows, produced p-values indicating a highly significant decline since the early 1990s. Consequently, dry catchments in late autumn, even with average cool season rainfall, have reduced dam inflows and decreased river heights downstream from Wagga Wagga, before water extraction for irrigation. It is concluded that lower April–September mean river heights at Wagga Wagga and decreased river height variability at Hay, since the mid-1990s, are due to combined lower April–May catchment precipitation and increased mean temperatures. Machine learning attribute detection revealed the southern MDB drivers as the southern annular mode (SAM), inter-decadal Pacific oscillation (IPO), Indian Ocean dipole (IOD) and global sea-surface temperature (GlobalSST). Continued catchment drying and warming will drastically reduce future water availability.