Shreedhar Maskey

Downscaling daily precipitation over the Yellow River source region in China: a comparison of three statistical downscaling methods

Three statistical downscaling methods are compared with regard to their ability to downscale summer (June–September) daily precipitation at a network of 14 stations over the Yellow River source region from the NCEP/NCAR reanalysis data with the aim of constructing high-resolution regional precipitation scenarios for impact studies. The methods used are the Statistical Downscaling Model (SDSM), the Generalized LInear Model for daily CLIMate (GLIMCLIM), and the non-homogeneous Hidden Markov Model (NHMM). The methods are compared in terms of several statistics including spatial dependence, wet- and dry spell length distributions and inter-annual variability. In comparison with other two models, NHMM shows better performance in reproducing the spatial correlation structure, inter-annual variability and magnitude of the observed precipitation. However, it shows difficulty in reproducing observed wet- and dry spell length distributions at some stations. SDSM and GLIMCLIM showed better performance in reproducing the temporal dependence than NHMM. These models are also applied to derive future scenarios for six precipitation indices for the period 2046–2065 using the predictors from two global climate models (GCMs; CGCM3 and ECHAM5) under the IPCC SRES A2, A1B and B1scenarios. There is a strong consensus among two GCMs, three downscaling methods and three emission scenarios in the precipitation change signal. Under the future climate scenarios considered, all parts of the study region would experience increases in rainfall totals and extremes that are statistically significant at most stations. The magnitude of the projected changes is more intense for the SDSM than for other two models, which indicates that climate projection based on results from only one downscaling method should be interpreted with caution. The increase in the magnitude of rainfall totals and extremes is also accompanied by an increase in their inter-annual variability.

Panta Rhei 2013–2015: global perspectives on hydrology, society and change

ABSTRACT In 2013, the International Association of Hydrological Sciences (IAHS) launched the hydrological decade 2013–2022 with the theme “Panta Rhei: Change in Hydrology and Society”. The decade recognizes the urgency of hydrological research to understand and predict the interactions of society and water, to support sustainable water resource use under changing climatic and environmental conditions. This paper reports on the first Panta Rhei biennium 2013–2015, providing a comprehensive resource that describes the scope and direction of Panta Rhei. We bring together the knowledge of all the Panta Rhei working groups, to summarize the most pressing research questions and how the hydrological community is progressing towards those goals. We draw out interconnections between different strands of research, and reflect on the need to take a global view on hydrology in the current era of human impacts and environmental change. Finally, we look back to the six driving science questions identified at the outset of Panta Rhei, to quantify progress towards those aims. Editor D. Koutsoyiannis; Associate editor not assigned

Spatio-temporal assessment of meteorological drought under the influence of varying record length: the case of Upper Blue Nile Basin, Ethiopia

Abstract This study investigates the spatial and temporal variation of meteorological droughts in the Upper Blue Nile (UBN) basin in Ethiopia using long historical records (1953–2009) for 14 meteorological stations, and relatively short records (1975–2009) for 23 other stations. The influence of using varying record length on drought category was studied by comparing the Standard Precipitation Index (SPI) results from the 14 stations with long record length, by taking out incrementally 1-year records from 1953 to 1975. These analyses show that the record length from 1953 to 1975 has limited effect on changing the drought category and hence the record length from 1975 to 2009 could be used for drought analysis in the UBN basin. Spatio-temporal analyses of the SPI values show that throughout the UBN basin seasonal or annual meteorological drought episodes occurred in the years 1978/79, 1984/85, 1994/95 and 2003/04. Persistency from seasonal to annual drought, and from one year to the next, has been found. The drought years identified by this SPI analysis for the UBN basin are known for their devastating impact in other parts of Ethiopia. Editor Z.W. Kundzewicz; Guest editor D. Hughes

Improved first-order second moment method for uncertainty estimation in flood forecasting

Abstract The first-order second moment (FOSM) method is widely used in uncertainty analysis. This method uses a linearization of the function that relates the input variables and parameters to the output variables. This simplification occasionally leads to problems when the mean value of the input variable is close to a local or global maximum or minimum value of the function. In this case, the FOSM computes artificially a zero uncertainty because the first derivative of the function is equal to zero. An improvement to the FOSM is proposed, whereby a parabolic reconstruction is used instead of a linear one. The improved FOSM method is applied to a flood forecasting model on the Loire River (France). Verification of the method using the Monte Carlo technique shows that the improved FOSM allows the accuracy of the uncertainty assessment to be increased substantially, without adding a significant burden in computation. The sensitivity of the results to the size of the perturbation is also analysed.

Rice yield in response to climate trends and drought index in the Mun River Basin, Thailand

Rice yields in Thailand are among the lowest in Asia. In northeast Thailand where about 90% of rice cultivation is rain-fed, climate variability and change affect rice yields. Understanding climate characteristics and their impacts on the rice yield is important for establishing proper adaptation and mitigation measures to enhance productivity. In this paper, we investigate climatic conditions of the past 30years (1984-2013) and assess the impacts of the recent climate trends on rice yields in the Mun River Basin in northeast Thailand. We also analyze the relationship between rice yield and a drought indicator (Standardized Precipitation and Evapotranspiration Index, SPEI), and the impact of SPEI trends on the yield. Our results indicate that the total yield losses due to past climate trends are rather low, in the range of <50kg/ha per decade (3% of actual average yields). In general, increasing trends in minimum and maximum temperatures lead to modest yield losses. In contrast, precipitation and SPEI-1, i.e. SPEI based on one monthly data, show positive correlations with yields in all months, except in the wettest month (September). If increasing trends of temperatures during the growing season persist, a likely climate change scenario, there is high possibility that the yield losses will become more serious in future. In this paper, we show that the drought index SPEI-1 detects soil moisture deficiency and crop stress in rice better than precipitation or precipitation based indicators. Further, our results emphasize the importance of spatial and temporal resolutions in detecting climate trends and impacts on yields.

Risk-Averse Economic Optimization in the Adaptation of River Dikes to Climate Change

To guarantee a safe flood defence in a changing environment, the adaptation to climate change needs to be considered in the design of river dikes. However, the large uncertainty in the projections of future climate leads to varied estimations of future flood probability. How to cope with the uncertainties in future flood probability under climate change is an inevitable question in the adaptation. In this paper, the uncertainty introduced by climate projections was integrated into the ‘expected predictive flood probability’, and the risk-aversion attitude was introduced in the adaptation of river dikes. The uncertainty of climate change impact on flood probability was represented by the uncertainty in the parameters of the probabilistic model. This parameter uncertainty was estimated based on the outputs from the GCMs participated in IPCC AR4. The parameter uncertainty estimated from different GCMs under selected scenarios was integrated into the expected predictive probability of flooding, which was used in the risk-averse economic optimization. Different optimal results were obtained based on varied values of the risk-aversion index. A case of dike ring area in China was studied as an example using the proposed approach. The results show that the uncertainty of climate change increases the optimal dike height and decreases the optimal safety level. The proposed approach enables decision makers to cope with the climate change and the associated uncertainty by adjusting the risk-aversion level.

Sediment related impacts of climate change and reservoir development in the Lower Mekong River Basin: a case study of the Nam Ou Basin, Lao PDR

This study applies the soil and water assessment tool (SWAT), with climate (precipitation and temperature) outputs from four general circulation models (GCMs) and a regional circulation model (PRECIS), to evaluate (1) the impacts of climate change on reservoir sedimentation and (2) the impacts of climate change and reservoir development on sediment outflow in the Nam Ou River Basin located in northern Laos. Three reservoir–density scenarios, namely one reservoir (1R), three reservoirs in series (3R), and five reservoirs in series (5R), were evaluated for both no climate change and climate change conditions. The results show that under no climate change conditions, by 2070, around 17, 14, and 15% of the existing reservoir storage volume in the basin will be lost for 1R, 3R, and 5R scenarios, respectively. Notably, under climate change scenario with highest changes in erosion and sediment outflux from the basin, the additional reduction in reservoir storage capacity due to sedimentation is estimated to be nearly 26% for 1R, 21% for 3R, and 23% for 5R. Climate change alone is projected to change annual sediment outflux from the basin by −20 to 151%. In contrast, the development of reservoirs in the basin will reduce the annual sediment outflux from the basin varying from 44 to 80% for 1R, 44–81% for 3R, and 66–89% for 5R, considering climate change. In conclusion, climate change is expected to increase the sediment yield of the Nam Ou Basin, resulting in faster reduction of the reservoir’s storage capacity. Sediment yield from the Nam Ou River Basin is likely to decrease significantly due to the trapping of sediment by planned reservoirs. The impact of reservoirs is much more significant than the impact of climate change on the sediment outflow of the basin. Hence, it is necessary to investigate appropriate reservoir sediment management strategies.

Hydrological Modeling for Drought Assessment

Droughts are widespread natural hazards and are reported to have killed more lives than all the other natural hazards combined over the past century. Repeated droughts can advance an area into desertification, severely impact on ecosystems, and cause many other tangible and intangible damages. Droughts can be characterized quantitatively for their spatial extent, intensity, and duration using various drought indices. In this chapter we focus on the assessment of various aspects of hydrology-related droughts and show that a process-based hydrological model has a lot to offer for drought assessment. To illustrate this we present a case study on the Limpopo river basin in Southern Africa. By comparing different types of droughts, we also highlight that various types of droughts need to be analyzed to understand the comprehensive nature of drought impacts.

Supporting well-informed decision and policy making through hydrological data analysis and modelling: the case of the Karkheh Basin, Iran

This paper shows how hydrological analysis and modelling can help guide water policy in the Karkheh Basin, Iran. The study suggests that water development strategy should take into account the high variability of the water resources, including declining low flows and intensification of floods in different parts of the basin. The modelling results suggest the promotion of soil and water conservation interventions due to their minimal impact on downstream flows. Conversion of rain-fed areas to irrigation may cause severe reduction in monthly flows. Therefore, only partial development is recommended, about 0.1 million ha out of a potential 0.5 million ha.

Estimating the Impacts and Uncertainty of Climate Change on the Hydrology and Water Resources of the Koshi River Basin

In this study, projections of temperature and precipitation in future periods and their impacts on hydrology and water resources of the Koshi River Basin in Nepal were investigated. The statistical downscaling model Long Ashton Research Station Weather Generator (LARS-WG) was used to downscale low-resolution data from ten general circulation models (GCMs) and three IPCC SRES scenarios (B1, A1B, and A2). The physically based hydrological model Soil and Water Assessment Tool (SWAT) was used to analyse the impacts of climate change on hydrology. LARS-WG simulated the baseline period (1981–2000) climate quite satisfactorily. Changes in climate and hydrological variables are presented at monthly and annual scales for three future periods: 2011–2030, 2046–2065, and 2080–2099. The results indicate that the Koshi basin tends to become warmer in the future as projected by all GCMs under three SRES scenarios. Changes in precipitation and streamflow are not univocal and vary depending on the GCM, GHGES. The difference in the projection of flow varies by as much as −35 to 51 % under the A1B scenario during the 2055s. The maximum increase in flow is projected during spring season with increase of 23 and 25 % during the 2055s and 2090s, respectively, under the A1B scenario. Similarly, the range of projections for all water balance components is very large. The water balance components: surface flow, baseflow, and water yield may decrease or increase in future periods, as GCMs do not agree on the direction of change. The potential ET and actual ET are projected to increase as projections from all GCMs and scenarios indicate, although a great deal of uncertainty exists in the magnitude of change. The potential ET is projected to increase in the range of 6–24 % in the 2090s. There is high variability among the models and scenarios for projections, and the variability increases with future time periods.