A nonlinear function approach for the normalized complementary relationship evaporation model

A nonlinear function approach for the normalized complementary relationship evaporation model that is different from the methodology maintaining the symmetric complementary relationship with appropriate definitions of potential and wet‐environment evaporation is proposed and verified. This approach employs the definitions used in the advection‐aridity model, wherein the potential is estimated using the Penman equation. Normalized by Penman potential evaporation, the complementary relationship model is expressed as a function describing the relationship between the evaporation ratio (the ratio of the actual to the Penman potential evaporation) and the proportion of the radiation term in Penman potential evaporation. The new nonlinear function proposed in the current study is approximately equivalent to the advection‐aridity and the modified Granger models under conditions that are neither too wet nor too dry, but is more reasonable under arid and wet conditions. The new nonlinear function model performs well in estimating actual evaporation, as verified by the observed data from four sites under different land covers. Copyright © 2011 John Wiley & Sons, Ltd.     

Integration of Penman approach with complementary principle for evaporation research

Natural evaporation occurs with water transportation from an unsaturated land surface into an unsaturated atmosphere. The subprocesses at the land surface and in the atmosphere are one‐sidedly emphasized in the Penman approach and the complementary principle, in which the ratio of actual evaporation to the Penman potential evaporation is expressed as a function of the wetness state of the land surface and the atmosphere, respectively. The Penman approach and complementary principle can be integrated for completely conceptualizing the evaporation process, by expressing the evaporation ratio as a function of both the land surface and atmospheric wetness. The integrated approach has the potential to increase the accuracy of evaporation estimation while reducing the burdens of parameterization.     

Spatial and temporal characteristics of actual evapotranspiration over Haihe River basin in China

Spatial and temporal characteristics of actual evapotranspiration over the Haihe River basin in China during 1960–2002 are estimated using the complementary relationship and the Thornthwaite water balance (WB) approaches. Firstly, the long-term water balance equation is used to validate and select the most suitable long-term average annual actual evapotranspiration equations for nine subbasins. Then, the most suitable method, the Pike equation, is used to calibrate parameters of the complementary relationship models and the WB model at each station. The results show that the advection aridity (AA) model more closely estimates actual evapotranspiration than does the Granger and Gray (GG) model especially considering the annual and summer evapotranspiration when compared with the WB model estimates. The results from the AA model and the WB model are then used to analyze spatial and temporal changing characteristics of the actual evapotranspiration over the basin. The analysis shows that the annual actual evapotranspirations during 1960–2002 exhibit similar decreasing trends in most parts of the Haihe River basin for the AA and WB models. Decreasing trends in annual precipitation and potential evapotranspiration, which directly affect water supply and the energy available for actual evapotranspiration respectively, jointly lead to the decrease in actual evapotranspiration in the basin. A weakening of the water cycle seems to have appeared, and as a consequence, the water supply capacity has been on the decrease, aggravating water shortage and restricting sustainable social and economic development in the region.     

Validity of the Bouchet’s complementary relationship at 102 observatories across China

Bouchet in 1963 hypothesized that for large homogeneous land surface with minimum advection of heat and moisture, there exists a 1:1 complementary relationship of potential and actual evaporation coupled through land-atmosphere feedbacks. The complementary relationship has been widely used to estimate regional actual evaporation and explain the pan evaporation paradox. We examine the standardized potential evaporation (potential evaporation divided by wet environment evaporation) at 102 observatories at different elevations across China. Generally, the relationship is appropriate at the low elevations (<1000 m). With the increase of elevation, vapor transfer power becomes much less than radiation energy budget because of lower vapor pressure deficit and stronger global solar radiation. As a result, at the high elevations (over 1000 m), the excess energy resulted by limited moisture availability is not enough to be converted into drying power of the air. This result suggests that the complementary relationship is asymmetric at the high elevations. Supported by the Presidential Special Award Foundation, the Chinese Academy of Sciences (Grant No. O7R70020SD) and the National Key Technology R & D Program (Grant No. 2006BAC08B0408)     

Complementary relationships for near-instantaneous evaporation

The complementary relationship between actual and potential evaporation provides evaporation (i.e. evapotranspiration) estimates from minimal data. Some versions that require a land surface temperature instead of a humidity measurement could potentially be used with routine remotely sensed surface temperature data. A comparison of alternative complementary approaches, including those that require land surface temperatures, was made at small (10–30 min) time scales with point measurements spatially, using data from the FIFE, CASES, SGP, and Sahel field experiments. The advection-aridity version and a related version based on the Penman and the Priestley–Taylor equations performed the best overall. One of the four versions that incorporated land surface temperature performed fairly well. The complementary approach appears to remain viable, especially in remote sensing applications with sparse data.     

A distributed approach for estimating catchment evapotranspiration: comparison of the combination equation and the complementary relationship approaches

In large river basins, there may be considerable variations in both climate and land use across the region. The evapotranspiration that occurs over a basin may be drastically different from one part of the region to another. The potential influence of these variations in evapotranspiration estimated for the catchment is weakened by using a spatially based distributed hydrological model in such a study. Areal evapotranspiration is estimated by means of approaches requiring only meteorological data: the combination equation (CE) model and the complementary relationship approach—the complementary relationship areal evapotranspiration (CRAE) and advection–aridity (AA) models. The capability of three models to estimate the evapotranspiration of catchments with complex topography and land‐use classification is investigated, and the models are applied to two catchments with different characteristics and scales for several representative years. Daily, monthly, and annual evapotranspiration are estimated with different accuracy. The result shows that the modified CE model may underestimate the evapotranspiration in some cases. The CRAE and AA models seem to be two kinds of effective alternatives for estimating catchment evapotranspiration. Copyright © 2003 John Wiley & Sons, Ltd.     

Estimation of annual actual evapotranspiration from nonsaturated land surfaces with conventional meteorological data

Land surface evapotranspiration is an important component both in earth surface heat and water bal-ance, on whose budgets weather and climate depend, to a great extent, for their changes are responsible for the formation and variation of vegetation features on the globe. Besides, the evapotranspiration is an im-portant topic of short-term flood forecasting and the estimation of runoff from mountainous sides. As a result, the problem as to the evapotranspiration has been one of the concerns in …     

Studies and evaluations on interception processes during rainfall based on a tank model

Some analyses are carried out with regard to canopy interception processes during rainfall events based on a tank model. A hypothesis, rainfall interception rate is proportional to the product of potential evaporation and rainfall intensity, is formed from past experimental data, and is applied to the data in this study. Computational equations are proposed to the interception rate and accumulative interception loss under constant rainfall intensity. Data from the Shirakawatani experimental forested catchment are used in order to examine the relationship between the interception rate and rainfall intensity, the ratio of the interception rate to rainfall intensity and potential evaporation, accumulative interception loss and the rainfall duration, and accumulative interception loss and accumulative rainfall. These regression relations show that interception processes are described by rainfall intensity and potential evaporation. An equation relating the aerodynamic resistance in the Penman–Monteith equation to rainfall intensity is proposed to explain the fact that the interception rate exceeds net radiation.     

A comparison of the hydrology of moorland under natural conditions,agricultural use and forestry

Abstract

Long term research has been conducted into the hydrological effects of different land usage of a wetland mire in southern Germany. Drainage for agriculture lowered the water table and reduced evaporation from about 110% of open water losses to just under the Penman short grass potential rate. The runoff regime was altered and peak flows increased. Afforestation of agricultural land increased evaporation losses to much higher levels than open water evaporation, and annual runoff was nearly halved. Forest growth reduced soil water and baseflows. Peak flows became smaller; the rate of reduction was particularly rapid in the early years of tree growth.     

Estimation of potential evapotranspiration in the mountainous Panama Canal watershed

Spatially distributed hydrometeorological and plant information within the mountainous tropical Panama Canal watershed is used to estimate parameters of the Penman–Monteith evapotranspiration formulation. Hydrometeorological data from a few surface climate stations located at low elevations in the watershed are complemented by (a) typical wet‐ and dry‐season fields of temperature, wind, water vapour and pressure produced by a mesoscale atmospheric model with a 3 × 3 km² spatial and hourly temporal resolution, and (b) leaf area index fields estimated over the watershed during a few years using satellite data with two different spatial and temporal resolutions. The mesoscale model estimates of spatially distributed surface hydrometeorological variables provide the basis for the extrapolation of the surface climate station data to produce input for the Penman–Monteith equation. The satellite information and existing digital spatial databases of land use and land cover form the basis for the estimation of Penman–Monteith spatially distributed parameter values. Spatially distributed 3 × 3 km² potential evapotranspiration estimates are obtained for the 3300 km² Panama Canal watershed. Estimates for Gatun Lake within the watershed are found to reproduce well the monthly and annual lake evaporation obtained from submerged pans. Sensitivity analysis results of potential evapotranspiration estimates with respect to cloud cover, dew formation, leaf area index distribution and mesoscale model estimates of surface climate are presented and discussed. The main conclusion is that even the limited spatially distributed hydrometeorological and plant information used in this study contributes significantly toward explaining the substantial spatial variability of potential evapotranspiration in the watershed. These results also allow the determination of key locations within the watershed where additional surface stations may be profitably placed. Copyright © 2006 John Wiley & Sons, Ltd.     

Multi‐model ensemble prediction of terrestrial evapotranspiration across north China using Bayesian model averaging

Using high‐quality dataset from 12 flux towers in north China, the performance of four evapotranspiration (ET) models and the multi‐model ensemble approaches including the simple averaging (SA) and Bayesian model average (BMA) were systematically evaluated in this study. The four models were the single‐layer Penman–Monteith (P–M) model, the two‐layer Shuttleworthe–Wallace (S–W) model, the advection–aridity (A–A) model, and a modified Priestley–Taylor (PT‐JPL). Based on the mean value of Taylor skill (S) and the regression slope between measured and simulated ET values across all sites, the order of overall performance of the individual models from the best to the worst were: S–W (0.88, 0.87), PT‐JPL (0.80, 1.17), P–M (0.63, 1.73) and A–A (0.60, 1.68) [statistics stated as (Taylor skill, regression slope)]. Here, all models used the same values of parameters, LAI and fractional vegetation cover as well as the forcing meteorological data. Thus, the differences in model performance were mainly attributed to errors in model structure. To the ensemble approach, the BMA method has the advantage of generating more skillful and reliable predictions than the SA scheme. However, successful implementation of BMA requires accurate estimates of its parameters, and some degradation in performance were observed when the BMA parameters generated from the training period were used for the validation period. Thus, it is necessary to explore the seasonal variations of the BMA parameters according the different growth stages. Finally, the optimal conditional density function of half‐hourly ET approximated well by the double‐exponential distribution. Copyright © 2016 John Wiley & Sons, Ltd.     

An objective assessment of soil-moisture deficit models

The performance of different soil-moisture deficit models was assessed by comparing model predictions with over 3000 neutron-probe observations of soil-moisture content at six grassland experimental sites operated by the Institute of Hydrology. The models were formulated using combinations of different equations for estimating potential evaporation and different regulating functions relating actual to potential evaporation via the moisture status of the soil. The inclusion of sophisticated evaporation equations (Priestley-Taylor, Penman, Thom-Oliver) gave no improvement in SMD prediction over a proposed simple evaporation formula requiring no direct meteorological measurements other than rainfall. The success of this formula demonstrates the conservative nature of annual potential evaporation within the U.K. both spatially and temporally and also suggests a possible natural feedback mechanism between atmospheric demand and grass transpiration.     

Evaporation estimation models for Lake Eğirdir,Turkey

Lake E?irdir is located in the Lakes District in southwestern Turkey and it is the second largest freshwater resource lake. Evaporation is an important parameter in hydrological and meteorological practical studies. This study has three objectives: (1) to develop models for the estimation of daily evaporation using measured data from the automated GroWeather meteorological station located near Lake E?irdir; (2) to compare the evaporation models with the classical Penman approach; (3) to evaluate the potential of each model. The comparisons are based on daily and monthly available data from 2001 and 2002. The evaporation estimation models (EEMs) developed in this paper have lower mean absolute errors and higher coefficient of determination R² values than the Penman method. In order to evaluate the potential of the EEMs, daily evaporation values are calculated by the Priestley–Taylor, Brutsaert–Stricker, de Bruin, Makkink and Hamon methods. The EEMs are statistically indistinguishable from the classical methods on the basis of the parameters of mean, standard deviation, etc. In the evaluation of daily and monthly values, the relative error percentage for daily evaporation has lower values than for monthly evaporation. It can be seen that the EEMs help in calculating daily evaporation rather than monthly. Final evaluation and comparison indicate that there is a good agreement between the results of EEMs and the Penman approach than with the classical methods. Copyright © 2006 John Wiley & Sons, Ltd.     

Aridity/humidity status of land surface in China during the last three decades

To understand regional status and differences is groundwork for researching environmental change, such as regional response to global change, land use/land-cover change, land desertification, and sand/dust storms. At present, geographers are search- ing for driving forces of environmental change and making efforts to reflect human actions on these changes[1―4]. To recognize regional difference, most researches focus on single factor, such as temperature, precipitation, soil and vegetation. Ho…     

A physically based distributed subsurface–surface flow dynamics model for forested mountainous catchments

This study was designed to develop a physically based hydrological model to describe the hydrological processes within forested mountainous river basins. The model describes the relationships between hydrological fluxes and catchment characteristics that are influenced by topography and land cover. Hydrological processes representative of temperate basins in steep terrain that are incorporated in the model include intercepted rainfall, evaporation, transpiration, infiltration into macropores, partitioning between preferential flow and soil matrix flow, percolation, capillary rise, surface flow (saturation‐excess and return flow), subsurface flow (preferential subsurface flow and baseflow) and spatial water‐table dynamics. The soil–vegetation–atmosphere transfer scheme used was the single‐layer Penman–Monteith model, although a two‐layer model was also provided. The catchment characteristics include topography (elevation, topographic indices), slope and contributing area, where a digital elevation model provided flow direction on the steepest gradient flow path. The hydrological fluxes and catchment characteristics are modelled based on the variable source‐area concept, which defines the dynamics of the watershed response. Flow generated on land for each sub‐basin is routed to the river channel by a kinematic wave model. In the river channel, the combined flows from sub‐basins are routed by the Muskingum–Cunge model to the river outlet; these comprise inputs to the river downstream. The model was applied to the Hikimi river basin in Japan. Spatial decadal values of the normalized difference vegetation index and leaf area index were used for the yearly simulations. Results were satisfactory, as indicated by model efficiency criteria, and analysis showed that the rainfall input is not representative of the orographic lifting induced rainfall in the mountainous Hikimi river basin. Also, a simple representation of the effects of preferential flow within the soil matrix flow has a slight significance for soil moisture status, but is insignificant for river flow estimations. Copyright © 2005 John Wiley & Sons, Ltd.     

Simulating the response of a closed‐basin lake to recent climate changes in tropical West Africa (Lake Bosumtwi,Ghana)

Historical changes in the level of Lake Bosumtwi, Ghana, have been simulated using a catchment‐scale hydrological model in order to assess the importance of changes in climate and land use on lake water balance on a monthly basis for the period 1939–2004. Several commonly used models for computing evaporation in data‐sparse regions are compared, including the Penman, the energy budget, and the Priestley–Taylor methods. Based on a comparison with recorded lake level variations, the model with the energy‐budget evaporation model subcomponent is most effective at reproducing observed lake level variations using regional climate records. A sensitivity analysis using this model indicates that Lake Bosumtwi is highly sensitive to changes in precipitation, cloudiness and temperature. However, the model is also sensitive to changes in runoff related to vegetation, and this factor needs to be considered in simulating lake level variations. Both interannual and longer‐term changes in lake level over the last 65 years appear to have been caused primarily by changes in precipitation, though the model also suggests that the drop in lake level over the last few decades has been moderated by changes in cloudiness and temperature over that time. Based on its effectiveness at simulating the magnitude and rate of lake level response to changing climate over the historical record, this model offers a potential future opportunity to examine the palaeoclimatic factors causing past lake level fluctuations preserved in the geological record at Lake Bosumtwi. Copyright © 2006 John Wiley & Sons, Ltd.     

High resolution water body mapping for SWAT evaporative modelling in the Upper Oconee watershed of Georgia,USA

Technological improvements in remote sensing and geographic information systems have demonstrated the abundance of artificially constructed water bodies across the landscape. Although research has shown the ubiquity of small ponds globally, and in the southeastern United States in particular, their cumulative impact in terms of evaporative alteration is less well quantified. The objectives of this study are to examine the hydrologic and evaporative importance of small artificial water bodies in the Upper Oconee watershed in the northern Georgia Piedmont, USA, by mapping their locations and modelling these small reservoirs using the Soil Water Assessment Tool. Comparative Soil Water Assessment Tool models were run with and without the inclusion of small reservoir surface area and volume. The models used meteorological inputs from 1990–2013 to represent years with drought, high precipitation, and moderate precipitation for both the calibration and evaluation periods. Statistical comparison of streamflow indicated that the calibration methodology produced results where the default model simulation without reservoirs fit observed flows more closely than the modified model with small reservoirs included (e.g., Nash–Sutcliffe efficiency of 0.72 vs. 0.64, r² of 0.73 vs. 0.66, and percent bias of 11.4 vs. 21.6). In addition, Penman–Monteith, Hargreaves, and Priestley–Taylor evapotranspiration equations were used to estimate actual evaporation from 2,219 small water bodies identified throughout the 1,936.8 km² watershed. Depending on the evaporation equation used, water bodies evaporated an average of 0.03–0.036 km³/year for the period 2003–2013. Using Penman–Monteith further, if the reservoirs were not considered and average actual evapotranspiration rates from the rest of the basin were applied, only 0.016 km³ of water would have left the basin as a result of evapotranspiration. This finding suggests construction of small reservoirs increased evaporation by an average of 0.017 km³ per year (approximately 46,500 m³/day). As the construction of small reservoirs continues and high resolution image data used to map these water bodies becomes increasingly available, watershed models that evolve to address the cumulative impacts of small water bodies on evaporation and other hydrologic processes will have greater potential to benefit the water resource management community.     

Non-dimensionalisation of the Penman-Monteith model

The Penman-Monteith model is expressed in non-dimensional form. Scalings are selected such that the groups of dimensionless variables lie between minus one and plus one. The relative importance of each term in the equation is assessed from the size of the dimensionless parameters. In extreme cases where very large or very small values are found, asymptotic analysis allows equation simplification.

Hourly field data from savannah vegetation and pine forest are applied to each model term in its new form. For pine forest, the equation describing evaporation rates can be reduced to a single term. Describing savannah evaporation, the aerodynamic conductance is modelled in two different ways and subsequent values of calibrated parameters within the bulk stomatal response function are found to be strongly dependent on this. The model dependence of such parameters is more easily understood with equations in normalised form.

A sensitivity analysis of the Penman-Monteith model to all driving variables is performed for both savannah and pine forest data.     

Wetland versus open water evaporation: An analysis and literature review

Is the total evaporation from a wetland surface (including: open water evaporation, plant transpiration and wet/dry soil evaporation) similar, lower, or higher than evaporation from an open water surface under the same climatic conditions? This question has been the subject of long debate; the literature does not show a consensus. In this paper we contribute to the discussion in two steps. First, we analyse the evaporation from a wetland with emergent vegetation (E_a) versus open water evaporation (E_w) by applying the Penman–Monteith equation to identical climate input data, but with different biophysical characteristics of each surface. Second, we assess the variability of measured E_a/E_w through a literature review of selected wetlands around the globe.We demonstrate that the ratio E_a/E_w is site-specific, and a function of the biophysical properties of the wetland surface, which can also undergo temporal variability depending on local hydro-climate conditions. Second, we demonstrate that the Penman–Monteith model provides a suitable basis to interpret E_a/E_w variations. This implies that the assumption of wetland evaporation to behave similar to open water bodies is not correct. This has significant implications for the total water consumption and water allocation to wetlands in river basin management.     

Assessing the impact of interannual variability of precipitation and potential evaporation on evapotranspiration

The impact of interannual variability of precipitation and potential evaporation on the long-term mean annual evapotranspiration as well as on the interannual variability of evapotranspiration is studied using a stochastic soil moisture model within the Budyko framework. Results indicate that given the same long-term mean annual precipitation and potential evaporation, including interannual variability of precipitation and potential evaporation reduces the long-term mean annual evapotranspiration. This reduction effect is mostly prominent when the dryness index (i.e., the ratio of potential evaporation to precipitation) is within the range from 0.5 to 2. The maximum reductions in the evaporation ratio (i.e., the ratio of evapotranspiration to precipitation) can reach 8–10% for a range of coefficient of variation (CV) values for precipitation and potential evaporation. The relations between the maximum reductions and the CV values of precipitation and potential evaporation follow power laws. Hence the larger the interannual variability of precipitation and potential evaporation becomes, the larger the reductions in the evaporation ratio will be. The inclusion of interannual variability of precipitation and potential evaporation also increases the interannual variability of evapotranspiration. It is found that the interannual variability of daily rainfall depth and that of the frequency of daily rainfall events have quantitatively different impacts on the interannual variability of evapotranspiration; and they also interact differently with the interannual variability of potential evaporation. The results presented in this study demonstrate the importance of understanding the role of interannual variability of precipitation and potential evaporation in land surface hydrology under a warming climate.