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Water flow patterns in the Mekong are changing because of on-going rapid hydropower development triggered by economic growth. Of immediate concern are the current and proposed hydropower dams in the transboundary Srepok, Sesan and Srekong (3S) Rivers, which contribute up to 20% of the Mekong's annual flows, have a large potential for energy production and provide critical ecosystem services to the downstream Tonle Sap Lake and Mekong Delta. The objective of this paper is to determine how the operation of the proposed largest individual dams and cascade dam schemes in the 3S Rivers will affect flow regimes and energy production. Daily flows were simulated over 20 years using the Soil and Water Assessment Tool and HEC-ResSim models for a range of dam development and operations scenarios. The development of all dams in the 3S basin under an operation scheme to maximize individual electricity production results in an average 98% increase in dry season flows at the 3S outlet. Over 55% of dry season flows changes are caused by seven proposed large dams, with the Lower Srepok 3 project causing the highest impact. The seven large dams will generate 33.0 GWh/day with a water volume of 17 679 x 106 m3, compared with the current and definite future dams generating 73.2 GWh/day with a much lower volume of 6616 x 106 m3. When a cascade of dams are operational, downstream dams with small reservoirs will produce more energy. However, the marginal increase in energy production from the development of additional dams in the 3S basin will decline rapidly relative to the required water storage increase,. Strategic decision-making on the future of each large proposed dam in the 3S basin needs to be considered by local governments after understanding cumulative operation effects and with further consideration to the potential impact on downstream ecosystem productivity and livelihoods. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

River tributaries have a key role in the biophysical functioning of the Mekong Basin. Of particular attention are the Sesan, Srepok, and Sekong (3S) rivers, which contribute nearly a quarter of the total Mekong discharge. Forty two dams are proposed in the 3S, and once completed they will exceed the active storage of China's large dam cascade in the upper Mekong. Given their proximity to the lower Mekong floodplains, the 3S dams could alter the flood-pulse hydrology driving the productivity of downstream ecosystems. Therefore, the main objective of this study was to quantify how hydropower development in the 3S would alter the hydrology of the Tonle Sap floodplain, the largest wetland in the Mekong and home to one of the most productive inland fisheries in the world. We coupled results from four numerical models representing the basin's surface hydrology, water resources development, and floodplain hydrodynamics. The scale of alterations caused by hydropower in the 3S was compared with the basin's definite future development scenario (DF) driven by the upper Mekong dam cascade. The DF or the 3S development scenarios could independently increase Tonle Sap's 30 day minimum water levels by 30 ± 5 cm and decrease annual water level fall rates by 0.30 ± 0.05 cm d-1. When analyzed together (DF + 3S), these scenarios are likely to eliminate all baseline conditions (1986-2000) of extreme low water levels, a particularly important component of Tonle Sap's environmental flows. Given the ongoing trends and large economic incentives in the hydropower business in the region, there is a high possibility that most of the 3S hydropower potential will actually be exploited and that dams would be built even in locations where there is a high risk of ecological disruptions. Hence, retrofitting current designs and operations to promote sustainable hydropower practices that optimize multiple river services -- rather than just maximize hydropower generation --- appear to be the most feasible alternative to mitigate hydropower-related disruptions in the Mekong. [ABSTRACT FROM AUTHOR]

The Mekong River supports unique biodiversity and provides food security for over 60 million people in the Indo-Burma region, yet potential changes to natural flow patterns from hydropower development are a major risk to the well-being of this system. Of particular concern is the ongoing and future development of 42 dams in the transboundary Srepok, Sesan and Sekong (3S) basin, which contributes up to 20% of the Mekong's annual flows and provides critical ecosystem services to the downstream Tonle Sap Lake and the Mekong Delta. To assess the magnitude of potential changes, daily flows were simulated over 20 years using the HEC ResSim and SWAT models for a range of dam operations and development scenarios. A 63% increase in dry season flows and a 22% decrease in wet season flows at the outlet of the 3S basin could result from the potential development of new dams in the main 3S rivers under an operation scheme to maximize electricity production. Water-level changes in the Mekong River from this scenario are comparable with changes induced by the current development of Chinese dams in the Upper Mekong Basin and are significantly higher than potential flow changes from the proposed 11 mainstream dams in the Lower Mekong Basin. Dams on the upper sub-tributaries of the 3S basin have very little effect on seasonal flow regimes because most of those projects are run-of-the-river dams and have small reservoir storages. The effects on hourly flow changes resulting from intra-daily reservoir operations, sediment movement, water quality, and ecology require further study. Strategic site selection and coordinated reservoir operations among countries are necessary to achieve an acceptable level of development in the basin and to mitigate negative effects in seasonal flow patterns, which sustain downstream ecosystem productivity and livelihoods. [ABSTRACT FROM AUTHOR]

The Mekong is one of the world's great rivers. It has the greatest mean annual flow in the world for a river basin of comparable size. The flow regime, with very distinct wet and dry seasons, supports a rich biodiversity and the world's largest freshwater fishery. Given that at the present time the hydrological regime of the Mekong remains in its natural state, the accelerating pace of water resources development will induce hydrological change. The natural productivity of the system is therefore potentially jeopardized. This paper reports the findings of simulation studies of the potential hydrological impacts of water resource development scenarios over future planning horizons. In the Definite Future scenario (next 5 years), the seasonal redistribution of water by on-going hydropower development will increase the dry season flow by 40-60% in the upper portion of the basin and by 20-30% in the Mekong Delta. The Foreseeable Future scenario (next 20 years) and Long-Term Future scenario (next 50 years) will result in relatively small changes to the flow regime as further increases in dry season reservoir releases will be offset by planned increases in irrigation and other consumptive water demands. All scenarios were predicted to reduce the average wet season flows by 4-14%, flow reversal to the Tonle Sap Lake by 7-16%, flooded areas by 5-8% and salinity intrusion areas in the Viet Nam Delta by 15-17%. Predicted changes in Definite Future scenario will be irreversible, necessitating improved coordination between the LMB countries and cooperation with China in order to manage the risks and maximize the regional benefits. The scenario assessments highlighted the areas where research is necessary to mitigate and manage impacts in order to ensure the reasonable and equitable use of the Mekong basin's water resources. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Reduction of uncertainty in hydrologic predictions is important for water resources development and management, especially in ungauged basins. The objective of this study was to predict rainfall and runoff in an ungauged basin located in a mountainous region of northern Thailand by using a weather radar to estimate rainfall and a modeling approach to simulate rainfall-runoff processes. The calibrated relationship () was used to estimate rainfall over a gauged basin and an assumed ungauged basin. The runoff simulations in the gauged and ungauged basins were conducted using the quasi-distributed hydrological model HEC-HMS. The simulation results in the gauged basin, for both calibration and verification, were in agreement with the observed data. Model parameters in the assumed ungauged basin were estimated using transposition and regionalization techniques. The predicted direct runoff hydrograph in the assumed ungauged basin using radar measured rainfall and the regionalization technique matches well with the measurements at the basin outlet. Based on the study results, the radar rainfall estimates and regionalization of the hydrologic model parameters are promising alternatives for improved hydrologic predictions in ungauged basins. [ABSTRACT FROM AUTHOR]

The present study develops a method called window correlation matching method (WCMM) to reduce collocation and timing errors in matching pairs of radar measured reflectivity, Ze, and gauge measured rainfall intensity, R, for improving the accuracy of the estimation of Ze--R relationships. This method was compared with the traditional matching method (TMM), the probability matching method (PMM) and the window probability matching method (WPMM). The calibrated relationship Ze=18.05R1.45 obtained from 7x7 km of space window and both present and 5 min previous time of radar observation for time window (S77T5) produces the best results for radar rainfall estimates for orographic rain over the Mae ChaemWatershed in the north of Thailand. The comparison shows that the Ze--R relationship obtained from WCMM provide more accuracy in radar rainfall estimates as compared with the other three methods. The Ze--R relationships estimated using TMM and PMM provide large overestimation and underestimation, respectively, of mean areal rainfall whereas WPMM slightly underestimated the mean areal rainfall. Based on the overall results, it can be concluded that WCMM can reduce collocation and timing errors in Ze--R pairs matching and improve the estimation of Ze--R relationships for radar rainfall. WCMM is therefore a promising method for improved radar-measured rainfall, which is an important input for hydrological and environmental modeling and water resources management. [ABSTRACT FROM AUTHOR]

The present study develops a method called window correlation matching method (WCMM) to reduce collocation and timing errors in matching pairs of radar measured reflectivity, Ze, and gauge measured rainfall intensity, R, for improving the accuracy of the estimation of Ze-R relationships. This method is compared with the traditional matching method (TMM) and the probability matching method (PMM). The relationship Ze=18.05R1.45 obtained from 7x7 km of space window and both present and 5 min previous time of radar observation for time window (S77T5) produces the best results for radar rainfall estimates for orographic rain over the Mae Chaem Watershed in north of Thailand. The comparison shows that the Ze-R relationships obtained from WCMM provide more accuracy in radar rainfall estimates as compared with the other two methods. The Ze-R relationships estimated using TMM and PMM show large overestimation and underestimation, respectively, of mean areal rainfall. Based on the overall results, it can be concluded that WCMM can reduce collocation and timing errors in Ze-R pairs matching and improve the estimation of Ze-R relationships for radar rainfall. WCMM is therefore a promising method for improved radar-measured rainfall, which is an important input for hydrological and environmental modeling and water resources management. [ABSTRACT FROM AUTHOR]