The project “Utilizing Stream Waters in the Suppression of Forest Fires with the Help of New Technologies” under the EU INTERREG IV “Black Sea Basin Joint Operational Programme 2007-2013”
Eco-Tiras International Association of River Keepers (NGO)
The translation of climate change projections into corresponding changes in runoff and streamflow through hydrological modeling are usually accompanied by additional uncertainties caused both by specific weaknesses encountered in hydrological models, and different external factors that cannot always be eliminated during models testing. Among such factors, the anthropogenic ones prevail. As a result, the modeling water yield from a watershed should be considered as a certain hypothetic runoff that reflects reality only for the “pristine environment”.
The movement and storage of water on watershed scales is a complex system affected by climatic, geologic, soil, land use, anthropogenic and other factors. The nature of processes inherent in surface and subsurface hydrology is best investigated by hydrologic models simulating these processes over different spatial and time intervals, and physiographical conditions. In recent years a number of conceptual hydrological models have been developed and increasingly used by researchers and water resource managers to understand and address the extensive array of water resource problems, including those related to sustainable water supply [5, 8]. In 2014-2015 the SWAT (Soil and Water Assessment Tool) [1, 9] model was used for the first time as a basis for investigations of Moldovan small rivers’ potential streamflow in current and likely future climates . As with any hydrological modeling, the SWAT application was preceded by its validation and calibration – two processes targeted at better adjusting of the model to local conditions in order to reduce its inherent uncertainty .
For the validation, the upper part of the Cogilnic River watershed (UCRW) − one of Moldova’s small rivers − was selected. This subwatershed forms a water yield from this river’s source to the Hincesti hydrological post where streamflow observations are carried out (Fig. 1). Thus, this project has compared the validation procedure to that of the UCRW modeled water yield with the observed streamflow. A three-year comparison saw (Table 1) that discrepancy between two indicators was very significant to be neglected. The observed annual streamflow, expressed as a percentage of the simulated water yield, amounts to only 9.9, 18.5 and 9.6 percents for 2010, 2011 and 2012 yrs, respectively. Moreover, these discrepancies could not be eliminated by standard procedures of SWAT calibration, supporting available experiences that capabilities of this model are quite limited in relation to small rivers, especially those exposed to great anthropogenic alterations. Really, according to Casac & Lalikin , the approximate reductions of the Cogilnic River streamflow due only partially to some anthropogenic factors are: land treatment – up to 20%; artificial reservoirs – 10-15%; irrigation – 4-5%; and urbanization – 10%. An inevitable bias in rivers streamflow modeling must be taken into account in all hydrological simulations.
But, on the other hand, such simulations can be useful for estimating the anthropogenic loading on water yields if they consider the observed discrepancy between modeled runoff and observed streamflow. This approach can be proposed for countries like Moldova where practically all small rivers are extremely polluted and shallow, being almost on the verge of extinction; here, precipitations and snow-melt water either evaporate or infiltrate to a different depth. Drying of small rivers seriously affects the general state of the watershed ecosystems, thus changing the plant cover and evapotranspiration. The dominance of evaporation over precipitation, coupled with intensive pollution, makes most small rivers unsuitable for any type of water use and strengthens the vulnerability to climate change of their basins as a whole .
Anthropogenic effects on the results of hydrological modeling should also be taken into account in assessing hydrologic consequences of climate change. In the foreseeable future, new values of air temperature and precipitation, which undoubtedly will affect a climate component of water balance, at the same time are unlikely to alter significantly other inputs to hydrological models, such as soils or slopes. Therefore, the above-discussed bias in quantitative estimations of water yields can be considered as a kind of ‘constant’. In this case the modeled hydrological projections based, for example, on SWAT simulations, being compared with their corresponding values in current climate could present likely changes of the latter.
The assessment of such changes in Moldova’s small rivers’ streamflow in the likely future climate was based on the latest high resolution data set from a multi-model ensemble of regional climate projections developed for the IPCC Fifth Assessment Report . These projections are also based on new approaches in accounting for greenhouse gas concentrations – the Representative Concentration Pathways (RCPs) – which assume the different targeted radiative forcing in the twenty first century . A SWAT modeling of the future runoff from watersheds of selected rivers, which used as weather inputs the new projections of local air temperature and precipitation, confirmed the IPCC assumptions about expected decrease of water resources (Table 2). In particular, a possible modeling reduction of water yields could reach from about 2% to 21% depending on a time horizon and radiative forcing. On average, this reduction can reach 6.5% in the 2021-2050s and 16% – in the last thirty years of the century. These estimations are very close to those that have been received in other works .
Vulnerability to climate change is a function of exposure and sensitivity to climate change impacts. Both these drivers of vulnerability, as applied to surface waters, depend heavily on environmental conditions in the river basins and expected trends in key climatic variables.
Climate change, hydrological modeling, streamflow, uncertainty
COUNTRIES OR REGIONS INVOLVED
The Republic of Moldova
Hydrologists, water experts, policy- and decision-makers dealing with water resource management, local communities
The Knowledge Platform is designed to promote and showcase an emerging set of approaches to water resources management that address climate change and other uncertainties -- increasing the use of "bottom-up approaches" through building capacity towards implementation, informing relevant parties, engaging in discussion, and creating new networks. This is an ongoing project of the Alliance for Global Water Adaptation (AGWA) funded by the World Bank Group.