Hydro-economic Modeling for Optimization of Sustainable Water Management of a River Basin in the Wake of Climate Change: Case Study of the Zarrine River Basin (ZRB), Iran

Food and water security will be a great future challenge due to the growth of population and unsustainable water usage. The renewable water resources in the MENA region including Iran, are expected to decline. This situation is exacerbated by the increasing demands and the rising climate variability in the wake of global climate change affecting adversely the available water resources. This thesis research attempts to evaluate the impacts of climate change and water demands on the hydrological and environmental characteristics of the Zarrine River Basin (ZRB), the headwater of Lake Urmia of Iran, which has been desiccating in recent years, being so on the brink of an environmental disaster, together with the Boukan Dam operation and then recommends some adaptation strategies, in order to balance properly the future water resources and demands in the ZRB. This thesis is divided in 6 chapters structured as 5 individual papers with the following contents: 1- In the first Chapter a general introduction of the thesis is given and the research area and study objectives are described. 2- In Chapter 2 the ZRB’s water resources response to the impacts of a changing climate, with emphasis on Boukan dam’s future inflow, are simulated up to year 2029 by means of the basin-wide hydrologic model SWAT. The future projections of precipitation and temperature extracted from MPI-ESM-LR-GCM (RCP 2.6 and 4.5) and HADCM3-GCM (SRES A2 and B2) output, respectively, are downscaled using Quantile Mapping (QM) bias-correction and SDSM, respectively. From two variants of QM employed, the Empirical-CDF-QM model decreases the biases of the raw GCM- precipitation the most. The SWAT- model is then calibrated and validated with historical (1981-2011) ZR-streamflow, using the SWAT-CUP- automatic program. The subsequent SWAT-simulations for the future 2012-2029 period indicate a reduction of the inflow to Boukan Dam as well as of the overall water yield of ZRB for all SRES/RPC- scenarios. In summary, based on these predictions and the expected increase of water demands, the ZRB is likely to face a water shortage in the future, with a water yield decrease between -23% and -36%, unless some adaptation plans are implemented for a better management of the water resources in the region. 3- Modeling the hydrologic responses to future changes of climate is important for improving adaptive water management. In Chapter 3 the impacts of future climate change signals on various hydro-climatic variables in the ZRB are assessed. To that avail, two SD-methods, SDSM and ECDF non-stationary Quantile Mapping (QM), are used to downscale the climate variables of min. and max. temperatures and precipitation for the near (2020-2038), middle (2050 to 2068) and far (2080 to 2098) future periods for three RCP emission scenarios. The performances of the downscaling methods are compared to each other for a past “future” period (2006-2016), and the QM is found to be better and is so selected in the subsequent SWAT- Zarrine-streamflow simulations, with the model calibrated and validated for the reference period (1991-2012). The impacts of climate change on the hydrologic response of the river basin, specifically, the inflow to the Boukan reservoir, the reservoir dependable water (DWR) supply are then compared for three RCPs in the three future periods, assuming (1) the “current” consumptive demand to be continued in the future and (2) a more conservative “recommended” one. A systematic future shortage of the available water is obtained for case (1) which can be mitigated somewhat for (2). Finally, the SWAT- predicted ZRB outflow is compared with the Montana-based estimated environmental flow of the ZR. The latter can successfully be sustained at good and fair levels for the near and middle future periods, but not so for the summer months of the far future period, particularly, for RCP85. 4- The operating policies of a reservoir dam depend on the upstream river’s inflow. As the latter may be affected by impacts of climate change which, in turn, may alter the water supply, studies to this regard are of utmost importance to ensure the sustainable development of the future water resources in the area. In Chapter 4, firstly the calibrated rainfall-runoff model SWAT is used to predict the various hydrological components of the ZRB for the historical and future periods until 2049, using downscaled input precipitation and temperatures of three RCP-scenarios captured from one of the projections of the CMIP5-GCM ensemble, the CESM1-CAM5-model. The streamflow scenarios are then employed in the MODSIM water management model to simulate the present-day and future water resources of the basin and to determine the optimal operation of the Boukan Dam, based on some prioritization of the water allocation. Finally, the impacts of climate and demand changes on the dam operation are evaluated by comparing future and historical MODSIM- simulated average water budget and supply/demand ratio (SDR) for the three CESM-model scenarios which show that the region will face more intensive water shortages. 5- The future water management policies in the study region should move toward more efficient and sustainable operating water supply systems, instead of just building new infrastructures. To evaluate such policies, in Chapter 5 an integrated hydro-economic tool is developed to integrate water supply and economic aspects of the Zarrine river basin and its irrigation network, supplied primarily from water of the Boukan dam. In this new model the future optimal water policies are investigated considering the hydrological, environmental and economic impacts of climate change, together with the land use change i.e. the agricultural crop patterns. The central concept of the hydro-economic model is that the water supply will be a function of the economic benefits for the water demanders, while respecting water resources system constraints. In the first step within the model, the surface and groundwater resources, especially, the inflow to the Boukan dam as well as the potential crop yields are simulated by SWAT, driven by GCM/QM-downscaled climate predictions under three climate RCPs for three future 21th – century periods. While in all 9 RPC/period- combinations consistently higher temperatures are predicted, the precipitation pattern are much more versatile, leading to corresponding changes in the future water yields. Using the MODSIM- water management tool, the SWAT- simulated water available is then optimally distributed across the different irrigation plots within the ZRB, while adhering to various environmental/demand priority constraints. MODSIM is subsequently coupled with the CSPSO-tool to optimize (maximize) the agro-economic water productivity (AEWP) of the various crops and, subsequently, the net economic benefits (NEB), using the crop areas as decision variables, while respecting various crop cultivation constraints. Adhering to political food security recommendations for the country, three variants of cereal cultivation area constraints are investigated. The results indicate considerably-augmented AEWPs, leading to a future increase of the annual NEB of ~16% to 37.4 Million USD for the 65%-cereal acreage variant, meanwhile, the irrigation water required is reduced by ~38%. This NEB- rise is achieved by augmenting the total future crop area in the ZRB by about 47% - indicating some deficit irrigation - wherefore most of this extension will be cultivated by the high AEWP- yielding crops wheat and barley, at the expense of a tremendous reduction of alfalfa acreage. Though covering today only small base acreages, depending on the future period/RCP, tomato- and, less so, potato- and sugar beet- cultivation areas will also be increased significantly. 6- In Chapter 6 temperatures and precipitation in the ZRB, predicted under RCP 4.5 and RCP8.5 for three 19-year-long future periods (2030, 2050 and 2080) by the HadGEM2-ES_RCA4, selected as the most suitable GCM-RCM model combination from the Coordinated Regional Downscaling Experiment (CORDEX) ensemble, are bias-corrected using the two step QM- method and then employed as input to the SWAT model to simulate the future ZRB water resources and, namely, the Boukan-reservoir inflows. Subsequently, the reservoir operation/water demands in the ZRB are modeled with the MODSIM water management tool for two water demand scenarios, WDcurrent and WDrecom, which represent the current and a more sustainable water demand scenario, respectively. The reliability of the dam’s water supply for different water uses in the study area is then investigated by computing the supply/ demand ratio (SDR). The results show that, although for WDrecom the SDRs are generally higher than for WDcurrent , the SDRs are all < 1, i.e. future water deficits still prevail. Finally, the performance of the water supply system is evaluated by means of risk, reliability, resiliency, vulnerability and maximum deficit indices and combining these to estimate its Sustainability Group Index (SGI). The findings indicate that, compared with the historical period, for both water demand scenarios, WDcurrent and WDrecom, the average SGI will be decreased significantly for each RCP, namely, for the more extreme RCP85 scenario. However, as expected, the SGI- decrease for WDrecom is less than that of WDcurrent, indicating indeed the advantage of implementing this more sustainable water demand scenario.