Projection of air temperature and evaporation form reservoirs under future climate change (Case study: Dez reservoir)

Reservoir storage is essential for developing dependable water supplies and is a major component of the river system water budget. The storage contents of reservoirs fluctuate greatly over time with variations in water use and hydrologic conditions that range from severe multiple-year droughts to floods. Water surface evaporation typically represents a major component of the reservoir water budget. Estimates of the amount and rate of evaporation from open water surfaces are required in water resource management for a variety of purposes, such as the design of storage reservoirs, catchment water balance studies, municipal and industrial water supply, irrigation of agricultural lands and management of wetlands. The impacts of reservoir evaporation on water management vary greatly based on location with differences in climate, reservoir characteristics, and water management and use practices. Determining evaporation rates is essential for efficient management of reservoirs and water resources, particularly in water-scarce countries such as Iran. It is estimated that open water reservoirs in Iran lose high volume of their total water storage capacity per year by evaporation. For example, this loss is around 13% of the volume of inflow to KHARKHEH Dam per year. While this loss is of significant concern, the threat of a changing climate has been directing greater focus to how much water will be lost from Iran’s reservoirs in the future. As a result of the change in climate, particularly the increase in surface air temperatures, evaporation is also expected to increase throughout Iran. This paper analyses evaporation rates from Dez dam reservoir for the two periods (2021-2050 and 2051-2080) under modeled climate change conditions using five General Climate Models (GCMs) with A2 emission scenario. Global climate models, also known as general circulation models (GCMs), are models which solve the primitive equations of mass, momentum and thermodynamics to generate a description of the state of the atmosphere, and produce most of the meteorological variables, such as wind speed, relative humidity, rainfall, surface air temperature and solar radiation. The GCMs are typically used together, as a multi-model analysis, where long time series into the future and different GHG emission scenarios are used in order to create climatic statistics. Inherent to GCMs, however, is their coarse spatial resolution which amongst all the available models varies from 200 to 500 km. With this resolution, albeit GCMs incorporate the important large-scale atmospheric circulation, they are unable to capture local-scale factors such as the orographic elevation, proximity to water bodies and local winds. Therefore, the outputs of GCMs need to downscale for applying to modeled evaporation. Daily meteorological prepared using K-NN downscaling model to estimate evaporation rate from open surface water using equations derived from penman equation. The K-NN downscaling model is a nonparametric resampling method for generating new data series. This method simulates daily precipitation for a weather station by combining a stochastic weather generator model (based on non-parametric K-Nearest Neighbor approach) with future climate scenarios that are established based on projected monthly. In this study, an equation based on penman equation for situations where no wind data are available is used to estimate evaporation rate. The equation is validated by compared to the penman equation in estimating the evaporation rate in a meteorological station that has the same climate as DEZ dam. Then, evaporated volume, which is estimated using area-capacity relations of Dez dam, were compared with observed meteorological variables for the period of 1982–2011. Area-capacity curves are usually used for reservoir flood routing, reservoir operation, determination of water surface area, and capacity corresponding to each elevation, reservoir classification, and reservoir sediment distribution. In this study, a dimensionless curve is used to estimate the changes of capacity in relation to the changes of evaporation rate. Results of the application of the presented model for downscaling monthly precipitation outputs of GCMs show that the model has a high capability for downscaling precipitation. So, the used equation to estimate the evaporation rate is suitable for applying in DEZ dam station. Also, the results showed that the volume of average annual evaporation from the study reservoir will increase about 7% in 2021-2050 and 10% in 2051-2080 more than base period. The main reason of this increase can be global warming in the future. According to the modelling results, the mean annual air temperature will be increased by 1.3C and 3C in 2021-2050 and 2051-2080 respectively. This will have a significant impact on the evaporation rates, especially in spring and summer which the increases in temperature will be too high.