Chong-Yu Xu

Cross Comparison of Empirical Equations for Calculating Potential Evapotranspiration with Data from Switzerland

Earlier studies (Singh and Xu, 1997; Xu and Singh, 2000, 2001) have evaluated and compared various popular empirical evapotranspiration equations that belonged to three categories:(1) mass-transfer based methods, (2) radiation based methods, and(3) temperature-based methods; and the best and worst equations of each category were determined for the study regions. In this study a cross comparison of the best or representative equation forms selected from each category was made. Five representativeempirical potential evapotranspiration equations selected from the three categories, namely: Hargreaves and Blaney-Criddle (temperature-based), Makkink and Priestley-Taylor (radiation-based) and Rohwer (mass-transfer-based) were evaluatedand compared with the Penman-Monteith equation using daily meteorological data from the Changins station in Switzerland.The calculations of the Penman-Monteith equation followed theprocedure recommended by FAO (Allen et al., 1998). Thecomparison was first made using the original constant valuesinvolved in each empirical equation and then made using therecalibrated constant values. The study showed that: (1) theoriginal constant values involved in each empirical equationworked quite well for the study region, except that the valueof α = 1.26 in Priestley-Taylor was found to be too high and therecalibration gave a value of α = 0.90 for the region.(2) Improvement was achieved for the Blaney-Criddle method by addinga transition period in determining the parameter k. (3) The differences of performance between the best equation forms selected from each category are smaller than the differences between different equations within each category as reportedin earlier studies (Xu and Singh, 2000, 2001). Further examinationof the performance resulted in the following rank of accuracy ascompared with the Penman-Monteith estimates: Priestley-Taylor andMakkink (Radiation-based), Hargreaves and Blaney-Criddle (temperature-based) and Rohwer (Mass-transfer).

Trend of estimated actual evapotranspiration over China during 1960-2002

[1] In this study, the water balance methodology introduced by Thornthwaite and Mather (1955) is modified to estimate monthly actual evapotranspiration for 686 stations over China during 1960–2002. The modification is done by replacing the Thornthwaite potential evapotranspiration estimation with the Penman-Monteith method. Temporal trend and spatial distribution of the estimated annual actual evapotranspiration during the past 43 years are analyzed. The results show that (1) the actual evapotranspiration had a decreasing trend in most areas east of 100E, and there was an increasing trend in the west and the north parts of northeast China; (2) the spatial distribution of the trend for the actual evapotranspiration is similar to that of the potential evapotranspiration in south China, while the trends are opposite in north China; (3) for most parts of China, the change in precipitation played a key role for the change of estimated actual evapotranspiration, while in southeast China, the change of potential evapotranspiration appeared to be the major factor; and (4) in general, the hydrological cycle was intensified in western China, whereas it was weakened from the Yellow River basin northward.

A Review on Monthly Water Balance Models for Water Resources Investigations

Research on the development and application of monthly water balance models has been carried out since the 1940s. A good body of experience has been gained for many models, and a review of these models is needed. Beginning with the development of monthly water balance models from the earliest times, this paper discusses the relevance of various aspects of the practical application of such models. Monthly water balance models were introduced originally to evaluate the importance of different hydrologic parameters under a variety of hydrologic conditions. Present applications of water balance models are directed along three main lines: reconstruction of the hydrology of catchments, assessment of climatic impact changes, and evaluation of the seasonal and geographical patterns of water supply and irrigation demand.

Modelling Hydrological Consequences of Climate Change—Progress and Challenges

The simulation of hydrological consequences of climate change has received increasing attention from the hydrology and land-surface modelling communities. There have been many studies of climate-change effects on hydrology and water resources which usually consist of three steps: (1) use of general circulation models (GCMs) to provide future global climate scenarios under the effect of increasing greenhouse gases, (2) use of downscaling techniques (both nested regional climate models, RCMs, and statistical methods) for “downscaling” the GCM output to the scales compatible with hydrological models, and (3) use of hydrologic models to simulate the effects of climate change on hydrological regimes at various scales. Great progress has been achieved in all three steps during the past few years, however, large uncertainties still exist in every stage of such study. This paper first reviews the present achievements in this field and then discusses the challenges for future studies of the hydrological impacts of climate change.

Evaluation and generalization of radiation‐based methods for calculating evaporation

Eight radiation-based equations for determining evaporation were evaluated and expressed in five generalized forms. Five evaporation equations (Abtew, Hargreaves, Makkink, Priestley and Taylor and Turc), where each represents one generalized form, were then compared with pan evaporation measured at Changins station in Switzerland. The comparison was first made using the original constant values involved in each equation, and then using the recalibrated constant values. Evaluation of the Priestley and Taylor equation requires net radiation data as input, in this study, net radiation was estimated using Equation (16) owing to the lack of observation data. The results showed that when the original constant values were used, large errors resulted for most of the equations. When recalibrated constant values were substituted for the original constant values, four of the five equations improved greatly, and all five equations performed well for determining mean annual evaporation. For seasonal and monthly evaporation, the Hargreaves and Turc equations showed a significant bias, especially for cold months. With properly determined constant values, the Makkink and modified Priestley and Taylor equations resulted in monthly evaporation values that agreed most closely with pan evaporation in the study region. The simple Abtew equation can also be used when other meteorological data except radiation are not available. Copyright

EVALUATION AND GENERALIZATION OF 13 MASS-TRANSFER EQUATIONS FOR DETERMINING FREE WATER EVAPORATION

Thirteen equations based on the mass-transfer method for determining free water evaporation were expressed in seven generalized equations. These seven equations were then compared with pan evaporation at four climatological stations in north-western Ontario, Canada. The comparisons were based on monthly evaporation. Equations were compared by calibrating them on the entire data sets as well as by calibrating on part of the data and then verifying them on the remainder of the data. The results of comparison showed that all equations were in reasonable agreement with observed evaporation, and that the eAect of wind velocity on monthly evaporation was marginal. However, when an equation with parameters obtained at one site was applied to compute evaporation at another site, the computed evaporation was not in good agreement with observed values. #1997 by John Wiley & Sons, Ltd.

Modelling the effects of climate change on water resources in central Sweden

This article describes investigationsinto the effects of climate change on flow regimes oftwenty-five catchments (from 6 to 1293 km2) incentral Sweden. Hydrological responses of fifteenhypothetical climate change scenarios (e.g.combinations of ΔT = +1, +2 and +4 °C andΔP = 0, ± 10%, ± 20%) were simulated by a conceptual monthly water balance model. The results suggest thatall the hypothetical climate change scenarios wouldcause major decreases in winter snow accumulation.Significant increase of winter flow and decrease ofspring and summer runoff were resulted from mostscenarios. Attendant changes in actualevapotranspiration were also examined for all climatechange scenarios. Despite the changes in seasonaldistribution of evapotranspiration, the change inannual total evapotranspiration was relatively smallwith the maximum change of 23% compared with the 76%for mean annual snow water equivalent changes and 52%for mean annual runoff changes. Such hydrologicresults would have significant implications on futurewater resources design and management.

Assessing uncertainties in a conceptual water balance model using Bayesian methodology

Abstract Abstract The aim of this study was to estimate the uncertainties in the streamflow simulated by a rainfall–runoff model. Two sources of uncertainties in hydrological modelling were considered: the uncertainties in model parameters and those in model structure. The uncertainties were calculated by Bayesian statistics, and the Metropolis-Hastings algorithm was used to simulate the posterior parameter distribution. The parameter uncertainty calculated by the Metropolis-Hastings algorithm was compared to maximum likelihood estimates which assume that both the parameters and model residuals are normally distributed. The study was performed using the model WASMOD on 25 basins in central Sweden. Confidence intervals in the simulated discharge due to the parameter uncertainty and the total uncertainty were calculated. The results indicate that (a) the Metropolis-Hastings algorithm and the maximum likelihood method give almost identical estimates concerning the parameter uncertainty, and (b) the uncertainties in the simulated streamflow due to the parameter uncertainty are less important than uncertainties originating from other sources for this simple model with fewer parameters.

Dependence of evaporation on meteorological variables at different time-scales and intercomparison of estimation methods

This paper consists of two parts. In the first part, the significance of five major factors, including solar radiation, vapour pressure deficit, relative humidity, wind speed and air temperature, that control evaporation were evaluated comparatively at diAerent time-scales using the data from Changines station in Switzerland. The comparative evaluation was made at hourly, daily, 10-dayand monthly time-scales. It was found that the role of controlling variables in evaporation varied with the time-scale. The vapour pressure deficit was best correlated with pan evaporation at all time-scales, while the wind speed was least correlated with pan evaporation, especially when the time period was longer than a day. In the second part, four equations for calculating evaporation, including temperature-based methods, humidity-based methods, mass transfer methods and radiation-based methods, were compared with pan evaporation. Of these four equations, the Penman equation, representing the mass transfer method, resulted in monthly evaporation values that agreed most closely with pan evaporation values. The Romanenko equation, representing the humidity method, also compared reasonably well with pan evaporation. The Turc equation, representing the radiation method, and the Thornthwaite equation, representing the temperature method, were found to underestimate evaporation significantly, especially for cold months. #1998 John Wiley & Sons, Ltd.

Regional water balance modelling in the NOPEX area: development and application of monthly water balance models

Abstract One of the main purposes of a water balance study is to evaluate the net available water resources, both on the surface and in the subsurface. Water balance models that simulate hydrographs of river flow on the basis of available meteorological data would be a valuable tool in the hands of the planners and designers of water resources systems. In this paper, a set of simple monthly snow and water balance models has been developed and applied to regional water balance studies in the NOPEX area. The models require as input monthly areal precipitation, monthly long-term average potential evapotranspiration and monthly mean air temperature. The model outputs are monthly river flow and other water balance components, such as actual evapotranspiration, slow and fast components of river flow, snow accumulation and melting. The results suggest that the proposed model structure is suitable for water balance study purposes in seasonally snow-covered catchments located in the region.