Effects of climate change and human interactions on water balance dynamics in the River Vistula basin

<div> <p>Water balance modelling is often applied in studies of climate and human impacts on water resources. Annual water balance is usually derived based on precipitation, discharge and temperature observations under an assumption of negligible changes in annual water storage in a catchment. However, that assumption might be violated during very dry or very wet years. In this study we apply groundwater level measurements to improve water balance modelling in nine sub-catchments of the River Vistula basin starting from the river sources downstream. Annual and inter-annual water balance is studied using a Budyko framework to assess actual evapotranspiration and total water supply. We apply the concept of effective precipitation to account for possible losses due to water interception by vegetation. Generalised Likelihood Uncertainty Estimation GLUE is used to account for parameter and structural model uncertainty, together with the application of eight Budyko-type equations. Seasonal water balance models show large errors for winter seasons while summer and annual water balance models follow the Budyko framework. The dryness index is much smaller in winter than in summer for all sub-catchments. The spatial variability of water balance modelling errors indicate an increasing uncertainty of model predictions with an increase in catchment size. The results show that the added information on storage changes in the catchments provided by groundwater level observations largely improves model accuracy. The results also indicate the need to model groundwater level variability depending on external factors such as precipitation and evapotranspiration and human interventions. The modelling tools developed will be used to assess future water balance in the River Vistula basin under different water management scenarios and climate variability.</p> </div>

Survival of the Qaidam mega-lake system under mid-Pliocene climates and its restoration under future climates

The Qaidam Basin in the north of the Tibetan Plateau has undergone drastic environmental changes during the last millions of years. During the Pliocene, the Qaidam Basin contained a freshwater mega-lake system although the surrounding regions showed increasingly arid climates. With the onset of the Pleistocene glaciations, lakes began to shrink and finally disappeared almost completely. Today, hyperarid climate conditions prevail in the low-altitude parts of the Qaidam Basin. The question of how the mega-lake system was able to withstand the regional trend of aridification for millions of years has remained enigmatic so far. This study reveals that the mean annual water balance, i.e. the mean annual change in terrestrial water storage in the Qaidam Basin, is nearly zero under present climate conditions due to positive values of net precipitation in the high mountain ranges and shows positive annual values during warmer, less dry years. This finding provides a physically based explanation for how mid-Pliocene climates could have sustained the mega-lake system and that near-future climates not much different from present conditions could cause water storage in reservoirs, raising lake levels and expanding lake areas, and may even result in restoration of the mega-lake system over geological timescales. The study reveals that a region discussed as being an analogue to Mars due to its hyperarid environments is at a threshold under present climate conditions and may switch from negative values of long-term mean annual water balance that have prevailed during the last 2.6 million years to positive ones in the near future.

Orographic Effects of Geomorphology on Precipitation in a Pluvial Basin of the Eastern Tibetan Plateau

The eastern Tibetan Plateau is subjected to strong spatial variations in precipitation, but the underlying reasons are still not well understood due to sparse in-situ meteorological observations. In this study, streamflow observations were adopted to investigate the orographic controls on precipitation in the Qingyijiang (QYJ) Basin of the eastern Tibetan Plateau. The method of multi-year annual water balance was used to estimate the basin-level precipitation using in-situ streamflow and flux-based evapotranspiration. In addition, elevation transect was designed to examine the possible links between precipitation and geomorphology. The results showed the severe under-estimation of regional precipitation by weather sites (~1150 mm yr−1) in the QYJ Basin, where the runoff depth was as high as ~1450 mm yr−1. The water balance revealed a much higher level of precipitation (~2000 mm yr−1) in the QYJ Basin, but precipitation in the two adjacent basins was contrastingly low (<1000 mm yr−1). The spatial pattern of precipitation was well consistent with the local horn-mouth geomorphology, with more precipitation occurring in the geomorphologically converging and elevating region. Furthermore, within the the QYJ Basin, annual precipitation was larger in the sub-basins (>2200 mm) on or near the bottom of the horn-mouth geomorphology than the others (<1800 mm). With these results, we concluded that the high precipitation level in the QYJ Basin could be attributed to the combined converging and lifting effects of geomorphology on the westward atmospheric vapor. Therefore, flooding risk should be carefully accounted for in the basins with similar geomorphology in the eastern Tibetan Plateau.

Shift of annual water balance in the Budyko space for catchments with groundwater-dependent evapotranspiration

The Budyko framework represents the general relationship between the evapotranspiration ratio (F) and the aridity index (φ) for the mean annual steady-state water balance at the catchment scale. It is interesting to investigate whether this standard F − φ space can also be applied to capture the shift of annual water balance in catchments with varying dryness. Previous studies have made significant progress in incorporating the storage effect into the Budyko framework for the non-steady conditions, whereas the role of groundwater-dependent evapotranspiration was not investigated. This study investigates how groundwater-dependent evapotranspiration causes the shift of the annual water balance in the standard Budyko space. A widely used monthly hydrological model, the ABCD model, is modified to incorporate groundwater-dependent evapotranspiration into the zone with a shallow water table and delayed groundwater recharge into the zone with a deep water table. This model is applied in six catchments in the Erdos Plateau, China, to estimate the actual annual evapotranspiration. Results show that the variations in the annual F value with the aridity index do not satisfy the standard Budyko formulas. The shift of the annual water balance in the standard Budyko space is a combination of the Budyko-type response in the deep groundwater zone and the quasi-energy limited condition in the shallow groundwater zone. Excess evapotranspiration (F > 1) could occur in dry years, which is contributed by the significant supply of groundwater for evapotranspiration. Use of groundwater for irrigation can increase the frequency of the F > 1 cases.