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This paper summarizes the initial results (first 4 yr) of a whole‐catchment manipulation to evaluate the impact of a loss of hydrologic connectivity of a boreal lake with its watershed on lake thermal and evaporative regimes. We diverted the upstream flow to a fourth order lake resulting in a marked reduction (81%) in watershed area, and compared the response to a comparable control lake in a headwater position. The manipulation reduced runoff into the experimental lake from 2400 mm to 90 mm, greatly increased theoretical residence time (from 2.3 yr to 18.3 yr), and reduced average dissolved organic carbon concentration from 5.8 ± 0.3 mg L−1 to 5.5 ± 0.3 mg L−1. Average Secchi depth was increased by 0.6 m, resulting in a 0.25 m deeper epilimnion and cooler surface water temperatures (8% of the time) than those predicted by pre‐manipulation relationships. There was some evidence that the response of evaporation to episodic events was altered, but this was not extensive enough to alter annual evaporation or median evaporation rates. Our findings show that the impacts of hydrologic connectivity on lake chemistry can cascade to alter the energy budgets of boreal lakes. These results can inform how differences in hydrological connectivity across the landscape due to lake‐watershed topology and climate will impact boreal lakes. Of note is that very clear lakes in the boreal region are highly sensitive to impacts from a change in clarity. Information on their spatial distribution will be necessary to assess impacts of hydrological connectivity on the boreal lake complex. [ABSTRACT FROM AUTHOR]

This paper examines the hydrological impact of the seasonality of precipitation and maximum evaporation: seasonality is, after aridity, a second-order determinant of catchment water yield. Based on a data set of 171 French catchments (where aridity ranged between 0.2 and 1.2), we present a parameterization of three commonly-used water balance formulas (namely, Turc-Mezentsev, Tixeront-Fu and Oldekop formulas) to account for seasonality effects. We quantify the improvement of seasonality-based parameterization in terms of the reconstitution of both catchment streamflow and water yield. The significant improvement obtained (reduction of RMSE between 9 and 14% depending on the formula) demonstrates the importance of climate seasonality in the determination of long-term catchment water balance. [ABSTRACT FROM AUTHOR]

This paper presents a neural network model capable of catchment-wide simultaneous prediction of river stages at multiple gauging stations. Thirteen meteorological parameters are considered in the input, which includes rainfall, temperature, mean relative humidity and evaporation. The NARX model is trained with a representative set of hourly data, with optimal time delay for both the input and output. The network trained using 120-day data is able to produce simulations that are in excellent agreement with field observations. We show that for application with one-step-ahead predictions, the loss in network performance is marginal. Inclusion of additional tidal observations does not improve predictions, suggesting that the river stage stations under consideration are not sensitive to tidal backwater effects despite the claim commonly made.EDITOR D. Koutsoyiannis ASSOCIATE EDITOR F. Pappenberger [ABSTRACT FROM PUBLISHER]

Summary: In a globally warming climate, observed rates of atmospheric evaporative demand have declined over recent decades. Several recent studies have shown that declining rates of evaporative demand are primarily governed by trends in the aerodynamic component (primarily being the combination of the effects of wind speed (u) and atmospheric humidity) and secondarily by changes in the radiative component. A number of these studies also show that declining rates of observed near-surface u (termed ‘stilling’) is the primary factor contributing to declining rates of evaporative demand. One objective of this paper was to review and synthesise the literature to assess whether stilling is a globally widespread phenomenon. We analysed 148 studies reporting terrestrial u trends from across the globe (with uneven and incomplete spatial distribution and differing periods of measurement) and found that the average trend was −0.014ms−1 a−1 for studies with more than 30 sites observing data for more than 30years, which confirmed that stilling was widespread. Assuming a linear trend this constitutes a −0.7ms−1 change in u over 50years. A second objective was to confirm the declining rates of evaporative demand by reviewing papers reporting trends in measured pan evaporation (E pan) and estimated crop reference evapotranspiration (ET o); average trends were −3.19mma−2 (n =55) and −1.31mma−2 (n =26), respectively. A third objective was to assess the contribution to evaporative demand trends that the four primary meteorological variables (being u; atmospheric humidity; radiation; and air temperature) made. The results from 36 studies highlighted the importance of u trends. We also quantified the sensitivity of rates of evaporative demand to changes in u and how the relative contributions of the aerodynamic and radiative components change seasonally over the globe. Our review: (i) shows that terrestrial stilling is widespread across the globe; (ii) confirms declining rates of evaporative demand; and (iii) highlights the contribution u has made to these declining evaporative rates. Hence we advocate that assessing evaporative demand trends requires consideration of all four primary meteorological variables (being u, atmospheric humidity, radiation and air temperature). This is particularly relevant for long-term water resource assessment because changes in u exert greater influence on energy-limited water-yielding catchments than water-limited ones. [ABSTRACT FROM AUTHOR]

Abstract: The Syrdarya River basin in Central Asia exhibits symptoms of scarcity because of growing competition between hydropower upstream and environment, cities and agriculture downstream. Different approaches to water management, based on water savings in agriculture could relieve the stress of competition, yet keep alive a vibrant agriculture. The Fergana Valley, a highly productive area within the upstream of the Syrdarya River, was analyzed to determine the water savings potential of agriculture. This paper used a water accounting procedure to identify both the scope for savings and the strategies to realize those gains. Significant nonproductive depletion of water at 2681–4164millionm3 (Mm3) was identified in the form of evaporation, flows to sinks and pollution. The water-saving potential is estimated at 2823Mm3 annually, which is about 10% of the total inflow into the area, through a range of practical approaches. This strategy would bring regional benefits by reallocation of surplus winter flow from the upstream for summer use in the downstream. [Copyright &y& Elsevier]

Spatial distribution of reference evapotranspiration (ET0) is essential in water resources planning and management, especially in semi-arid areas. In this paper, a digital elevation model is used in an `interpolate-then-calculate' approach to calculating the spatially distributed ET0 using the physically based Penman-Monteith equation in the Taoer river basin in China. The results show the following. (1) Of 11 interpolation methods, the Inverse Distance Weighting (IDW) method was found to be best for interpolating wind speed and a tn-variate secondary trend surface method was found most suitable for interpolating mean air temperature and relative humidity. Spatial modelling of the radiation environment considered the effects of elevation, slope and aspect. (2) Monthly values in January for the three meteorological variables showed larger spatial variations than in July, and just the reverse of net surface radiation. (3) The resulting ET0 calculated at each grid cell with 200m resolution and its spatial variation showed strong seasonal variation. Lower ET0 was found in high-elevation southern Great Xingan mountains in the northwest basin, while higher values were located in the plains adjacent to the lower reach. (4) The ET0 distribution by the `interpolate-then-calculate' approach better reflected the effects of topography than that of the `calculate-then-interpolate' approach. [ABSTRACT FROM AUTHOR]

Abstract: The nonparametric Mann–Kendall test was used to detect the trends of major hydroclimatic variables in the Tarim River Basin, the largest inland river basin in China for the period of 1960–2007. Results showed that both mean annual air temperature and precipitation experienced an increasing trend, while annual streamflow demonstrated a mixed trend of decreasing and increasing: The mountainous region upstream showed an increasing trend and the region downstream exhibited a decreasing trend. Impacts of the increased air temperature on streamflow have shown different characteristics depending on location and seasons: it has positive effect on the runoff at mountainous region due to snowmelt and glacier-melt in spring, but negative effect on the runoff at plain area due to the increase of actual evaporation in summer. In addition, human activity contributed to the declining of streamflow in the arid plain oases at downstream of the Tarim River Basin. The results obtained in this paper can be used as a reference for the planning and management of water resources to maintain the health of the river system. [Copyright &y& Elsevier]

Summary: Many water resource investigations require an estimate of evaporation from water supply reservoirs. Despite its recognised limitations, the pan coefficient method for estimating reservoir evaporation is widely used in industry due to its moderate data requirements. This paper presents a framework to quantify the uncertainties associated with estimates of reservoir evaporation generated using the pan coefficient method. The uncertainties associated with each of the inputs are quantified using a combination of statistical techniques, including Bayesian statistics and subjective judgement; before being combined using Monte Carlo simulation. The framework is applied to three reservoirs in the Werribee River catchment, Australia. The 95% probability intervals surrounding the estimates of reservoir evaporation are as large as ±40% of the best estimate. The largest contributing factor to uncertainty is shown to be the estimation of pan evaporation at reservoirs without a climate station, followed by the uncertainty surrounding the annual pan coefficient itself. The assessment shows that the largest reductions in uncertainty can be achieved by installing an evaporation pan at the reservoir. [Copyright &y& Elsevier]

Information on evapotranspiration (ET) can help us understand water balance, particularly in forested watersheds. Previous studies in China show that ET was relatively low (30-40% of total precipitation) in the Minjiang Valley located in the upper reach of the Yangtze River Basin. However, this conclusion was derived from research on small-scale watersheds (<100 km2). The objective of this paper was to present ET information on meso-scale watersheds in the Minjiang Valley. Four meso-scale watersheds (1,700-5,600 km2) located in the Minjiang Valley were used to estimate ET using the water balance approach. We first generated forest vegetation variables (coniferous forest percentage, forest cover percentage, and derived forest vegetation index) using remote sensing data. Landsat 5 TM satellite images, acquired on June 26, 1994, were selected for the vegetation classification. Actual annual ET was calculated based on 11-year estimated precipitation and measured streamflow data (1992-2002). We also calculated potential ET (PET) using an improved Thornthwaite model for all four watersheds for the period of 1992-1998. PET can provide additional information about potential capacity of water flux to atmosphere in the region. Seasonal (dry and rainy) PET and ET for all studied watersheds were also estimated for comparison purposes as the water balance approach, at shorter than annual scales, would likely provide inaccurate estimates of ET. The dominant vegetations in the Minjiang Valley were grasslands, conifer forests, and shrub-lands. Our results confirmed that both ET and PET for three studied meso-scale watersheds in the Minjiang Valley is relatively low (39.5-43.8 and 28.2-47.7% for ET and PET, respectively), with an exception of ET in the Yuzixi watershed being 71.1%. This result is generally consistent with previous research at small watershed scales. Furthermore, the low ET across various scales in the Minjiang Valley may be related to the unique deeply cut valley environment. [ABSTRACT FROM AUTHOR]

Natural forests in southern China have been severely logged due to high human demand for timber, food, and fuels during the past century, but are recovering in the past decade. The objective of this study was to investigate how vegetation cover changes in composition and structure affected the water budgets of a 9.6-km2 Dakeng watershed located in a humid subtropical mountainous region in southern China. We analyzed 27 years (i.e., 1967-1993) of streamflow and climate data and associated vegetation cover change in the watershed. Land use/land cover census and Normalized Difference of Vegetation Index (NDVI) data derived from remote sensing were used to construct historic land cover change patterns. We found that over the period of record, annual streamflow ( Q) and runoff/precipitation ratio did not change significantly, nor did the climatic variables, including air temperature, Hamon’s potential evapotranspiration (ET), pan evaporation, sunshine hours, and radiation. However, annual ET estimated as the differences between P and Q showed a statistically significant increasing trend. Overall, the NDVI of the watershed had a significant increasing trend in the peak spring growing season. This study concluded that watershed ecosystem ET increased as the vegetation cover shifted from low stock forests to shrub and grasslands that had higher ET rates. A conceptual model was developed for the study watershed to describe the vegetation cover-streamflow relationships during a 50-year time frame. This paper highlighted the importance of eco-physiologically based studies in understanding transitory, nonstationary effects of deforestation or forestation on watershed water balances. [ABSTRACT FROM AUTHOR]

Summary: Evaporation is an important part of the water balance of a catchment, and estimates of potential evaporation (PE) are an important input to hydrological models. When modelling the hydrological impacts of climate change, using data from climate models, present models generally do not provide direct estimates of PE from the land surface. Estimates thus have to be calculated from other climate variables, and many existing formulae can be applied. This paper compares the well-established, more physically based but data-intensive, Penman–Monteith PE against a simple, temperature-based (T-based) PE, when calculated from readily available monthly climate model data over Britain (for five global and eight regional climate models). The performance of the two PE formulations is compared to MORECS PE (a gridded dataset derived by the UK Met Office from weather observations, using a modified Penman–Monteith formulation) for the baseline period 1961–1990, and the changes in the two PE estimates between the 1970s and the 2080s are compared. The results show that the T-based PE matches MORECS PE better than does Penman–Monteith PE, for all the climate models studied. However, the changes in the two types of PE between the 1970s and 2080s are different, for each of the climate models, and these affect the modelled hydrological impacts. This is illustrated using three example catchments spread across Britain. The uncertainty introduced by the PE formulation is less than that due to the climate model, but could still be important for some applications. [Copyright &y& Elsevier]

This psper presents the development and characteristics of a site-specific physically based, spatially distributed model. The Grote Nete catchment model was built to simulate the various processes of the hydrological cycle, and their interactions with each other. It highlights the various stages of model development from conceptualisation and the underlying mathematical representation of hydrological processes, through the model set-up phase, and finally to the calibration and validation. MIKE SHE was chosen for its ability to generate a spatially distributed representation of the hydrological processes within a catchment, which was necessary for future scenario analysis. The equations coupled within the system include a three-dimensional Darcy equation in the saturated zone, the Richards equation for the unsaturated zone, the diffusion wave equation for real overland flow and the fully dynamic Saint Venant equations for flow in channels. Evapotranspirstion from the surface and subsurface are modelled using land cover and climatic factors to define the complete water budget using a physically based formulation. The paper presents the results of a multi-criteria evaluation protocol that included graphical and statistical testing, which involved the use of seven data comparison plots. Results of model calibration and validation against pre-established criteria are presented and discussed. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents the results of a hydrogeochemical study of the Lake Junin watershed, Peru, to evaluate the controls over chemical composition of water in this remote Andean site. Stream, spring, and lake waters in the Lake Junin basin are all Ca2+ and HCO3 − type. Evaporation has enriched the δ18O of the lake water by about 6‰ compared to that of input water. The 87Sr/86Sr ratio of dissolved strontium varies from 0.70777 to 0.72242 in the watershed because of water interactions with limestone and silicate rocks, respectively, in the watershed. The 87Sr/86Sr ratios of strontium in water from streams that flow through clastic sedimentary rocks east of the lake is more radiogenic (>0.7100) than elsewhere in the watershed, where the 87Sr/86Sr ratios of surface waters range between 0.7076 and 0.7090. These lower ratios are consistent with marine limestone sources found in these watersheds. Synoptic discharge measurements, combined with the strontium isotopic data, show that that three rivers provide over 90% of the surface water to the lake: the Río Chacachimpa, the Río San Juan, and the Río Palcamayo and about 60% of the total water input to the lake, the rest coming from direct precipitation. [Copyright &y& Elsevier]