ALBERT

Description: Globally, there have been many extreme weather events in recent decades. A challenge has been to determine whether these extreme weather events have increased in number and intensity compared to the past. This challenge is made more difficult due to the lack of long-term instrumental data, particularly in terms of river discharge, in many regions including Southeast Asia. Thus our main aim in this paper is to develop a river basin scale approach for assessing interannual hydrometeorological and discharge variability on long, palaeological, time scales. For the development of the basin-wide approach, we used the Mekong River basin as a case study area, although the approach is also intended to be applicable to other basins. Firstly, we derived a basin-wide Palmer Drought Severity Index (PDSI) from the Monsoon Asia Drought Atlas (MADA). Secondly, we compared the basin-wide PDSI with measured discharge to validate our approach. Thirdly, we used basin-wide PDSI to analyse the hydrometeorology and discharge of the case study area over the study period of 1300–2005. For the discharge-MADA comparison and hydrometeorological analyses, we used methods such as linear correlations, smoothing, moving window variances, Levene type tests for variances, and wavelet analyses. We found that the developed basin-wide approach based on MADA can be used for assessing long-term average conditions and interannual variability for river basin hydrometeorology and discharge. It provides a tool for studying interannual discharge variability on a palaeological time scale, and therefore the approach contributes to a better understanding of discharge variability during the most recent decades. Our case study revealed that the Mekong has experienced exceptional levels of interannual variability during the post-1950 period, which could not be observed in any other part of the study period. The increased variability was found to be at least partly associated with increased El Niño Southern Oscillation (ENSO) activity.

Description: River tributaries have a key role in the biophysical functioning of the Mekong Basin. Of particular interest 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, together with definite future (DF) plans for infrastructure development through the basin, would alter the hydrology of the Tonle Sap's 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 DF scenario 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 day−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 be exploited and that dams will be built even in locations where there is a high risk of ecological disruption. 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.

Description: The transboundary Mekong River is facing two ongoing changes that are expected to significantly impact its hydrology and the characteristics of its exceptional flood pulse. The rapid economic development of the riparian countries has led to massive plans for hydropower construction, and projected climate change is expected to alter the monsoon patterns and increase temperature in the basin. The aim of this study is to assess the cumulative impact of these factors on the hydrology of the Mekong within next 20–30 yr. We downscaled the output of five general circulation models (GCMs) that were found to perform well in the Mekong region. For the simulation of reservoir operation, we used an optimisation approach to estimate the operation of multiple reservoirs, including both existing and planned hydropower reservoirs. For the hydrological assessment, we used a distributed hydrological model, VMod, with a grid resolution of 5 km × 5 km. In terms of climate change's impact on hydrology, we found a high variation in the discharge results depending on which of the GCMs is used as input. The simulated change in discharge at Kratie (Cambodia) between the baseline (1982–1992) and projected time period (2032–2042) ranges from −11% to +15% for the wet season and −10% to +13% for the dry season. Our analysis also shows that the changes in discharge due to planned reservoir operations are clearly larger than those simulated due to climate change: 25–160% higher dry season flows and 5–24% lower flood peaks in Kratie. The projected cumulative impacts follow rather closely the reservoir operation impacts, with an envelope around them induced by the different GCMs. Our results thus indicate that within the coming 20–30 yr, the operation of planned hydropower reservoirs is likely to have a larger impact on the Mekong hydrograph than the impacts of climate change, particularly during the dry season. On the other hand, climate change will increase the uncertainty of the estimated reservoir operation impacts: our results indicate that even the direction of the flow-related changes induced by climate change is partly unclear. Consequently, both dam planners and dam operators should pay closer attention to the cumulative impacts of climate change and reservoir operation on aquatic ecosystems, including the multibillion-dollar Mekong fisheries.

Description: The Tonle Sap Lake of Cambodia is the largest freshwater body of Southeast Asia, forming an important part of the Mekong River system. The lake has an extremely productive ecosystem and operates as a natural floodwater reservoir for the lower Mekong Basin, offering flood protection and assuring the dry season flow to the Mekong Delta. In light of the accelerating pace of water resources development within the Mekong Basin and the anticipation of potentially significant hydrological impacts, it is critical to understand the overall hydrologic regime of Tonle Sap Lake. We present here a detailed water balance model based on observed data of discharges from the lake's tributaries, discharge between Mekong and the lake through the Tonle Sap River, precipitation, and evaporation. The overland flow between the Mekong and lake was modelled with the EIA 3D hydrodynamic model. We found that majority (53.5%) of the water originates from the Mekong mainstream, but the lake's tributaries also play an important role contributing 34% of the annual flow, while 12.5% is derived from precipitation. The water level in the lake is mainly controlled by the water level in the Mekong mainstream. The Tonle Sap system is hence very vulnerable, from a water quantity point of view, to possible changes in the Mekong mainstream and thus, development activities in the whole Mekong basin. From a biogeochemical point of view, the possible changes in the lake's own catchment are equally important, together with the changes in the whole Mekong Basin. Based on our findings, we recommend of continuing the monitoring programmes in lake's tributaries and urgently starting of groundwater measurement campaign within the floodplain, and including the groundwater modelling to be part of the hydrodynamic models applied for the lake. © 2013 John Wiley & Sons, Ltd.

Description: Global-scale water issues such as its availability, water needs or stress, or management, are mapped at various resolutions and reported at many scales, mostly along political or continental boundaries. As such, they ignore the fundamental heterogeneity of hydroclimates and natural boundaries of river basins. Here we describe the continental landmasses at two levels: eight hydrobelts strictly limited by river basins, defined at a 30' (0.5°) resolution, which are decomposed on continents as 26 hydroregions. The belts were defined and delineated, based primarily on the annual average temperature (T) and run-off (q), to maximise inter-belt differences and minimise intra-belt variability. This new global puzzle defines homogeneous and near-contiguous entities with similar hydrological and thermal regimes, glacial and postglacial basin histories, endorheism distribution and sensitivity to climate variations. The mid-latitude, dry and subtropical belts have northern and southern analogues and a general symmetry can be observed for T and q between them. The boreal and equatorial belts are unique. Population density between belts and between the continents varies greatly, resulting in pronounced differences between the belts with analogues in both hemispheres. Hydroregions (median size 4.7 M km2) are highly contrasted, with the average q ranging between 6 and 1393 mm yr−1 and the average T between −9.7 and +26.3 °C, and a population density ranging from 0.7 to 0.8 p km−2 for the North American boreal region and some Australian hydroregions to 280 p km−2 for some Asian hydroregions. The population/run-off ratio, normalised to a reference pristine region, is used to map and quantify the global population at risk of severe water quality degradation. Our initial tests suggest that hydrobelt and hydroregion divisions are often more appropriate than conventional continental or political divisions for the global analysis of river basins within the Earth system and of water resources. The GIS files of the hydrobelts and hydroregions are available at the supplement of this article and at doi:10.1594/PANGAEA.806957 as well as geotypes.net.

Description: During recent decades the Mekong River has experienced substantial interannual variations between droughts and major floods. The causes of these variations have been sought in climate change and dam construction. However, so far little research has addressed whether these recent variations are significantly different to long-term variations in the past. Hence, the aim of our paper is to place the recent variations between droughts and floods into a historical and paleoclimatological context. To achieve this we analysed the Mekong's meteorological conditions over the period 1300–2005 with a basin scale approach by using the Monsoon Asia Drought Atlas (MADA), which is a Palmer Drought Severity Index (PDSI) dataset derived from tree-ring growth records. The correlation analyses, both in time and frequency domains, showed correlation between MADA and the Mekong's discharge over the period 1910–2005 which suggests that MADA can be used as proxy for the hydrometeorology of the Mekong Basin. We found that the meteorological conditions of the Mekong varied at multi-annual, decadal and centennial scales over the study period. We found two especially distinct features: firstly, multi-annual and decadal variation between prolonged wet and dry epochs; and secondly, epochs with higher or lower interannual variability between very dry and wet years. Furthermore we found two epochs with exceptionally large interannual variability, one at the beginning of 17th century and the other in the post 1950 epoch. Both epochs are characterized by distinct increases in variability between very wet and dry years. The variability in the post 1950 epoch is much higher compared to any of the other epochs included in this study. Thus, during recent decades the climate in the Mekong has exhibited features that have not been experienced for at least several centuries. These findings call for further climate research, particularly regarding increased climate variability, and resilient adaptation and development approaches in the basin.

Description: Interannual climatic and hydrologic variability has been substantial during the past decades in many regions. While climate variability and its impacts on precipitation and soil moisture have been rather intensively studied, less is known on its impacts on freshwater availability and further implications for global food production. In this paper we quantify effects of hydroclimatic variability on global "green" and "blue" water availability and demand in agriculture. Analysis is based on climate forcing data for the past 30 yr with demography, diet composition and land use fixed to constant reference conditions. We thus assess how observed interannual hydroclimatic variability impacts on the ability of food production units (FPUs) to produce a given diet for their inhabitants, here focused on a benchmark for hunger alleviation (3000 kilocalories per capita per day, with 80% vegetal food and 20% animal products). We applied the LPJmL vegetation and hydrology model to calculate spatially explicitly the variation in green-blue water availability and the water requirements to produce that very diet. An FPU was considered water scarce if its water availability was not sufficient to produce the diet (neglecting trade from elsewhere, i.e. assuming food self-sufficiency). We found that altogether 24% of the global population lives in areas under chronic scarcity (i.e. water is scarce every year) while an additional 19% live under occasional water scarcity (i.e. water is scarce in some years). Of these 2.6 billion people under some degree of scarcity, 55% would have to rely on international trade to reach the reference diet while for 24% domestic trade would be enough (assuming present cropland extent and management). For the remaining 21% of population under scarcity, local food storage and/or intermittent trade would be enough secure the reference diet over the occasional dry years.

Description: Irrigation intensifies land use by increasing crop yield but also impacts water resources. It affects water and energy balances and consequently the microclimate in irrigated regions. Therefore, knowledge of the extent of irrigated land is important for hydrological and crop modelling, global change research, and assessments of resource use and management. Information on the historical evolution of irrigated lands is limited. The new global Historical Irrigation Dataset (HID) provides estimates of the temporal development of the area equipped for irrigation (AEI) between 1900 and 2005 at 5 arc-minute resolution. We collected subnational irrigation statistics from various sources and found that the global extent of AEI increased from 63 million ha (Mha) in 1900 to 112 Mha in 1950 and 306 Mha in 2005. We developed eight gridded versions of time series of AEI by combining subnational irrigation statistics with different data sets on the historical extent of cropland and pasture. Different rules were applied to maximize consistency of the gridded products to subnational irrigation statistics or to historical cropland and pasture data sets. The HID reflects very well the spatial patterns of irrigated land in the western United States as shown on historical maps. Mean aridity on irrigated land increased and river discharge decreased from 1900–1950 whereas aridity decreased from 1950–2005. The dataset and its documentation are made available in an open data repository at https://mygeohub.org/publications/8 (doi:10.13019/M2MW2G).

Description: Floods are amongst the most dangerous natural hazards in terms of economic damage. Whilst a growing number of studies have examined how river floods are influenced by climate change, the role of natural modes of interannual climate variability remains poorly understood. Here, we present the first global assessment of the influence of El Niño Southern Oscillation (ENSO) on river floods. The analysis was carried out by simulating daily gridded discharges using the WaterGAP model, and examining statistical relationships between these discharges and ENSO indices. We found that, over the period 1958–1999, ENSO exerted a significant influence on annual floods in river basins covering over a third of the world's land surface, and that its influence on floods has been much greater than its influence on average flows. We show that there are more areas in which annual floods intensify with La Niña and decline with El Niño than vice versa. However, we also found that in many regions the strength of the relationships between ENSO and annual floods have been non-stationary, with either strengthening or weakening trends during the study period. We discuss the implications of these findings for science and management. Given the strong relationships between ENSO and annual floods, we suggest that more research is needed to assess relationships between ENSO and flood impacts (e.g. loss of lives or economic damage). Moreover, we suggest that in those regions where useful relationships exist, this information could be combined with ongoing advances in ENSO prediction research, in order to provide year-to-year probabilistic flood risk forecasts.

Description: Many large basins in the “Monsoon Asia” region have sparse surface observation networks of the hydrometeorological parameters needed for hydrological modeling. These models are often used in water resources–related planning, impact assessments, and flood forecasting, which sets strict requirements for model accuracy and reliability. The aim of this study was to assess the performance of several publicly available reanalyses and remotely sensed datasets when used in modeling of discharges in the Mekong River basin. Tested precipitations were extracted from Tropical Rainfall Measuring Mission (TRMM) 3B42, versions 6 and 7; Asian Precipitation–Highly-Resolved Observational Data Integration Towards Evaluation of Water Resources (APHRODITE); Climate Forecast System Reanalysis (CFSR); and Interim ECMWF Re-Analysis (ERA-Interim) datasets. Temperature data were extracted from CFSR and ERA-Interim datasets. The model results obtained using these datasets were compared to measured discharges and modeled values based on daily surface observations. It was found that using TRMM, version 7, and APHRODITE precipitation datasets together with CFSR temperature data resulted in similar accuracy of computed discharges in the Mekong main stem as using surface observation data. This indicates that these gridded datasets might support well the modeling efforts in monsoon-driven large river basins in Monsoon Asia.