Evapotranspiration Components Research Articles

Representation of Terrestrial Hydrology and Large-Scale Drought of the Continental United States from the North American Regional Reanalysis

Abstract The North American Regional Reanalysis (NARR) is a state-of-the-art land–atmosphere reanalysis product that provides improved representation of the terrestrial hydrologic cycle compared to previous global reanalyses, having the potential to provide an enhanced picture of hydrologic extremes such as floods and droughts and their driving mechanisms. This is partly because of the novel assimilation of observed precipitation, state-of-the-art land surface scheme, and higher spatial resolution. NARR is evaluated in terms of the terrestrial water budget and its depiction of drought at monthly to annual time scales against two offline land surface model [Noah v2.7.1 and Variable Infiltration Capacity (VIC)] simulations and observation-based runoff estimates over the continental United States for 1979–2003. An earlier version of the Noah model forms the land component of NARR and so the offline simulation provides an opportunity to diagnose NARR land surface variables independently of atmospheric feedbacks. The VIC model has been calibrated against measured streamflow and so provides a reasonable estimate of large-scale evapotranspiration. Despite similar precipitation, there are large differences in the partitioning of precipitation into evapotranspiration and runoff. Relative to VIC, NARR and Noah annual evapotranspiration is biased high by 28% and 24%, respectively, and the runoff ratios are 50% and 40% lower. This is confirmed by comparison with observation-based runoff estimates from 1130 small, relatively unmanaged basins across the continental United States. The overestimation of evapotranspiration by NARR is largely attributed to the evapotranspiration component of the Noah model, whereas other factors such as atmospheric forcings or biases induced by precipitation assimilation into NARR play only a minor role. A combination of differences in the parameterization of evapotranspiration and in particular low stomatal resistance values in NARR, the seasonality of vegetation characteristics, the near-surface radiation and meteorology, and the representation of soil moisture dynamics, including high infiltration rates and the relative coupling of soil moisture with baseflow in NARR, are responsible for the differences in the water budgets. Large-scale drought as quantified by soil moisture percentiles covaries closely over the continental United States between the three datasets, despite large differences in the seasonal water budgets. However, there are large regional differences, especially in the eastern United States where the VIC model shows higher variability in drought dynamics. This is mostly due to increased frequency of completely dry conditions in NARR that result from differences in soil depth, higher evapotranspiration, early snowmelt, and early peak runoff. In the western United States, differences in the precipitation forcing contribute to large discrepancies between NARR and Noah/VIC simulations in the representation of the early 2000s drought.

Characteristics of canopy interception loss in Moso bamboo forests of Japan

AbstractIn recent years, Moso bamboo (Phyllostachys pubescens) forests have rapidly expanded in Japan by replacing surrounding coniferous and/or broadleaved forests. To evaluate the change in water yield from forested areas because of this replacement, it is necessary to examine evapotranspiration for Moso bamboo forests. However, canopy interception loss, one of the major components of evapotranspiration in forested areas, has been observed in only two Moso bamboo forests in Japan with relatively high stem density (~7000 stems/ha). There are, in fact, many Moso bamboo forests with much lower stem density. Thus, we made precipitation (Pr), throughfall (Tf) and stemflow (Sf) observations for 1 year in a Moso bamboo forest with stem density of 3611 stems/ha and quantified canopy interception loss (Ic). Pr and Ic for the experimental period were 1636 and 166 mm, respectively, and Ic/Pr was 10%. The value was approximately the same as values for the other two Moso bamboo forests and lower than values for coniferous and broadleaved forests. On the other hand, Tf/Pr and Sf/Pr for our forest (86% and 4%, respectively) were approximately 10% of Pr larger and smaller than values for the other two Moso bamboo forests. These results suggest that the difference in stem density greatly affects precipitation partitioning (i.e. Tf/Pr and Sf/Pr) but does not greatly change Ic/Pr. Copyright © 2012 John Wiley & Sons, Ltd.

Effect of urbanisation on the water balance of a catchment with shallow groundwater

Summary The impact of urbanisation on the water balance of a catchment dominated by surface water and groundwater interactions was investigated by using a process-based coupled surface water and groundwater model called MODHMS. The modelling estimated the likely changes in river discharge as a result of the land use change in the Southern River catchment in Western Australia. The catchment has both permeable soils and a shallow watertable. There was a significant increase in total annual discharge from the urbanised area where the runoff coefficient rose from 0.01 to more than 0.40. However in contrast with urban areas elsewhere these changes were mainly due to a shift in the subsurface water balance, including both groundwater and the unsaturated zone due to specifics of local hydrogeological conditions and adopted practice of storm runoff management. Due to the highly permeable soils, it is also common practice in the local building industry to direct runoff from roofs and roads into the soil and thereby the unconfined aquifer. Urbanisation results in particularly large changes in evapotranspiration from the soil profile and shallow watertable. The total subsurface evaporative flux reduced from 90% of infiltration (or 63–68% rainfall) to less than 29% (or 20% of rainfall) after urbanisation. Up to 83% (or 443 mm) of the pre-development evapotranspiration flux was from the shallow watertable. The requirement to control groundwater levels with drains in the shallow unconfined aquifer as well as the introduction of impervious surfaces caused a significant reduction of this component of evapotranspiration to less than 154 mm. These combined with an increase in infiltration rates, due to the direct infiltration of roof and road runoff, lead to higher groundwater recharge rates and subsequently groundwater discharge to the urban drainage network. The magnitude of urbanisation on catchment fluxes is most strongly influenced by urban density and the rate of local groundwater abstraction, which is used for urban irrigation. The analysis highlights that urban development in such areas reduces evaporation and evapotranspiration and therefore generates harvestable water. Depending on local needs this water could improve environmental flows and water for public and private water supply.

Estimation of Evapotranspiration and Water Budget Components Using Concurrent Soil Moisture and Water Table Monitoring

Simultaneous measurements of soil moisture profiles and water table heads, along a flow path, were used to determine evapotranspiration (ET) along with other components of the water budget. The study was conducted at a small-scale (~0.8 Km2) hydrologic monitoring field site in Hillsborough County, Florida, from January 2002 to June 2004. Frequency Domain Reflectometry soil moisture probes, installed in close proximity to water table monitoring wells were used to derive changes in the soil water storage. A one-dimensional transect model was developed; changes in the soil water storage and water table observations served as input to determine all vertical and lateral boundary fluxes along the shallow water table flow plane. Two distinct land cover environments, grassland and an alluvial wetland forest, were investigated in this particular study. The analysis provided temporally variable ET estimates for the two land covers with annual totals averaging 850 mm for grassland, to 1100 mm for the alluvial wetland forest. Quantitative estimates of other components of a water budget, for example, infiltration, interception capture, total rainfall excess, and runoff were also made on a quarterly and annual basis. Novelty of this approach includes ability to resolve ET components and other water budget fluxes that provide useful parameterization and calibration potential for predictive simulation models.

Assessment of the Regional Climate Model Version 3 over the Maritime Continent Using Different Cumulus Parameterization and Land Surface Schemes

Abstract This paper describes an assessment of the Regional Climate Model, version 3 (RegCM3), coupled to two land surface schemes: the Biosphere–Atmosphere Transfer System, version 1e (BATS1e), and the Integrated Biosphere Simulator (IBIS). The model’s performance in simulating precipitation over the Maritime Continent was evaluated against the 3-hourly Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis 3B42 product. It is found that the model suffers from three major errors in reproducing the observed rainfall histogram: underestimation of the frequency of dry periods, overestimation of the frequency of low-intensity rainfall, and underestimation of the frequency of high-intensity rainfall. Additionally, the model does not accurately reproduce the observed timing of the diurnal rainfall peak, particularly over land. These four errors persisted regardless of the choice of lateral boundary conditions, convective parameterization scheme, or land surface scheme. The magnitude of the wet–dry bias in the simulated volumes of rainfall was, however, strongly dependent on the choice of the convection scheme and lateral boundary conditions. The Grell convection scheme with Fritsch–Chappell closure was the best performing of the convection schemes, having the smallest error magnitudes in both the rainfall histogram and average diurnal cycle, and also having good representation of the land surface energy and evapotranspiration components. The 40-yr ECMWF Re-Analysis (ERA-40) was found to produce better simulations of observed rainfall when used as lateral boundary conditions than did the NCEP–NCAR reanalysis. Discussion of the nature of the major model errors is provided, along with some suggestions for improvement.

Quantifying evapotranspiration and its components in a coniferous subalpine forest in Southwest China

AbstractEvaporation is one of the most important processes in the soil‐plant‐atmosphere‐continuum water cycle. The objectives of the present study were to (1) test the feasibility of different methods for quantifying evapotranspiration (ET) and its components and (2) investigate and quantify ET and its components in a subalpine watershed from April to October, 2009. Our research site was in the Gongga Mountains, located on the southeastern fringe of the Qinghai‐Tibet Plateau. The components of ET could be observed using the interception, sap‐flow and stable isotope techniques. The summation of these components, referred to as the summed components method, was thought to be the ET. Similar estimates of ET from April to October were obtained using the summed components (736 mm) and Eddy covariance (598 mm) methods. The mean of two ET estimates (667 mm) accounted for 50% of the total water input. ET was composed of 6% soil evaporation, 19% vegetation transpiration and 75% interception evaporation. Copyright © 2011 John Wiley & Sons, Ltd.

森林与径流关系的模拟与分析

为了揭示森林与径流关系的空间变化及原因，用生物物理/动态植被模型SSiB4/TRIFFID与TOPMODEL的耦合模型SSiB4T/TRIFFID模拟了西南山区长江上游的梭磨河流域不同气温下植被演替及蒸发与径流的动态响应过程，分析了气温变化对森林–径流关系的影响及机理。控制试验模拟结果表明：1) 由于亚高山区气温低雨季冠层湿润分数高，在蒸散三个分量中植被蒸腾最小，森林蒸腾与草和灌木差异小，流域蒸散在植被从C3草到灌木演替阶段增加并达到最大值，在植被从灌木到森林演替阶段减小甚至低于裸土蒸发，因此增加了流域径流量。2) 温度较控制试验减小1.5℃，由于森林冠层蒸散减小幅度大于草和灌木使森林增加径流的作用增加。3) 气温较控制试验增加2.0℃，森林冠层截留蒸发较草和灌木明显增加，蒸腾也高于草和灌木，森林对径流的影响已不明显。当气温较控制试验增加5℃，森林蒸腾和冠层截留蒸发较草和灌木明显增加，森林覆盖流域总蒸发最大，森林减小了径流量。随着山区海拔高度下降，森林对径流的影响将发生从增加径流量到对径流量影响不大和减小径流量的变化。由于森林冠层温度和冠层阻力对气温变化比草和灌木敏感，导致对于相同的气温减小(增加)幅度森林冠层截留蒸发和蒸腾的减小(增加)幅度均大于草和灌木，通过这种机制气温变化造成森林与径流的关系发生变化。 In order to reveal the spatial variation of the relationship between forests and runoff and explore the mechanism numerical simulations of the responses of evaporation and runoff to the dynamic changes of vegetation over the Suomo basin (a tributary of the Yangtze River in the mountain region of southwestern China) are conducted under different temperature change scenarios by using the coupled model SSiB4T/ TRIFFID (Plant Dynamic Vegetation Model TRIFFID and TOPMODEL are integrated into the land surface model SSiB4). The impacts of temperature changes on forest-runoff relationship and the mechanism are ana-lyzed. The results of control test show that in wet season canopy wetness fraction is high in the subalpine re-gion of southwestern China duo to low temperature. Transpiration is the minimum among three components of evapotranspiration. Transpiration of forests is no significant difference with transpiration of grass and shrub. The evapotranspiration of the basin increases and reaches its maximal value during the period of C3 grass succession into shrub and then decreases during the period of shrub succession into forest and reaches its minimal value which may be less than bare soil evaporation. As a result forests increase the total runoff of the basin. A decrease in temperature by 1.5°C enhances such effect of forests duo to more decrease in water loss through canopy interception evaporation and transpiration of forests. An increase in temperature by 2°C enhances the rate of transpiration and evaporation of forests more than that of grass and shrub. As a result forests have no significant effect on runoff. Temperature rises to 5°C will cause forests turn to reduce runoff duo to more significant increase in water loss through canopy interception evaporation and transpiration of forests. As elevation decreases (temperature increases) in the mountain region of southwestern China the role of forests to increase runoff will change to be that forests have no significant effect on runoff and forests reduce runoff. The results also indicate that canopy temperature and canopy resistance of forests are more sensitive to temperature changes than that of grass and shrub. As a result canopy interception evaporation and transpiration of forests are more sensitive to temperature changes which cause that more significant increase or decrease in forest canopy evapotranspiration with temperature increase or decrease than that of grass and shrub. Through such mechanism changes of temperature cause the changes of forest-runoff relationship.

Inferring Transpiration Control from Sap Flow Heat Gauges and the Penman-Monteith Equation

In situ measurements of crop transpiration can enhance field studies of crop water use and productivity. Our objectives were to develop and evaluate a canopy resistance model, to evaluate operational characteristics of sap flow heat gauges (SFHG) for field corn (Zea mays L.), and to compare transpiration flux calculated by SFHG with evapotranspiration (ET) calculated by a Penman-Monteith (P-M) algorithm. Five sap flow heat gauges, controlled by an automated data acquisition system, were deployed in each of four replicated field plots of corn irrigated to limit water deficits to 50% of available water capacity at Garden City, Kansas, in 2004 and 2006. Water flux through each stem was estimated as a residual of a heat balance equation. Gauges were transferred to adjacent plants after 13 to 21 days to evaluate and mitigate functional stem damage. Loss of gauge operation, primarily due to stem damage, commonly occurred approximately one to two weeks following installation. Data screened using operational metrics were used to evaluate a scaling relationship between calculated flow to ET calculated following the P-M form. A canopy resistance model was derived from principles of radiation use. Transpiration calculated from gauge data was linearly related to ET calculated from the P-M, with R2 exceeding 0.79. Greater precision was obtained by assuming constant canopy resistance (rc), but predictive bias was reduced by assuming that rc was proportional to solar radiation. Sap flow gauges provided information useful for calibrating an rc model for P-M to calculate the transpiration component of ET; the model linking canopy resistance to absorbed radiation has application to dual-source and thermal-based energy balance models of crop ET.

Water use of grasslands, agroforestry systems and indigenous forests

The biotic and abiotic components of ecosystems affect each other through complex interactions and processes. These dynamic interactions give ecosystems their distinct identities and provide ecosystem services critical to human survival (e.g. water, energy and nutrients). However, human activities (e.g. commercial forestry, agriculture) have placed increasing demands on specific ecosystem services. The effect of these activities on ecosystem processes has been the focus of numerous Water Research Commission (WRC) studies. Some of these have determined man’s impact on plant-water use, biomass production (energy) and water use efficiency (biomass produced per unit of water transpired, termed productive green-water use). For example, measurements of evapotranspiration (ETa) from different vegetation types showed that annual water use is strongly related to the proportion of the year in which a dense canopy of transpiring leaves is maintained. Thus, evergreen vegetation such as riparian fynbos and plantations of introduced tree species exhibit a relatively high annual ETa, when compared to seasonal grasslands and deciduous trees that only maintain their transpiring canopy during summer. Quantification of the annual volumes of water used by these different vegetation types, under differing climatic and site conditions, has been possible through these studies. At a stand scale, measurements of the different components of evapotranspiration have allowed the partitioning of beneficial (transpiration) and non-beneficial (evaporation) fluxes. At a catchment scale measurements have quantified the proportional allocation of water to the different components of the water balance. Three case studies are presented to illustrate this. In a stand of Jatropha curcas, measurements of daily total evaporation rates during December to February (summer) on clear hot days ranged between 3 mm·d-1 to 4 mm·d-1. However, due to the deciduous nature of the species, water use was negligible (< 1 mm·d-1) during winter (May to August). At a catchment scale, studies in a montane grassland ecosystem of the KwaZulu-Natal Drakensberg showed that the partitioning of the main hydrological fluxes into streamflow and evaporation was dependent on the wetness of the hydrological year. In average to wet years (>1 200 mm precipitation) the hydrological flux was equally split between evaporation (650 mm) and runoff (550 mm), while in drier years evaporation became the dominating component of the water balance (752 mm vs. 356 mm, respectively). The data provided an important baseline for comparison with other impacted ecosystems (especially commercial forestry). Finally, results of a variety of studies on the growth and water use of indigenous trees growing in natural forest and plantation systems suggest that, compared to introduced tree species, indigenous species use substantially less water, show lower water use efficiency, and grow more slowly. Advantages to such indigenous systems potentially include lower management costs, higher product values, a wider range of non-wood products and a lower hydrological impact. Their usefulness may be greatest on sensitive sites (e.g. riparian zones, water-stressed catchments, land cleared of alien plants, land with a high erosion risk, degraded forest) where land use systems with a reduced environmental impact are required.Keywords: evaporation, transpiration, grassland, indigenous trees, Jatropha curcas

Contributions of imaging spectroscopy to improve estimates of evapotranspiration

AbstractImproved estimates of evapotranspiration (ET) are needed for water resource management and irrigation scheduling. We review the use of imaging spectroscopy to capture estimates of water vapour flux and biophysical components of ET. Remote sensing has long attempted to quantify and predict ET, with most applications relying only on green vegetation indexes from multispectral imagers combined with thermal radiance and weather data. In contrast, imaging spectrometry is an advanced remote sensing technology that measures hundreds of spectral bands in the solar spectrum. Plant pigments, water, and dry matter have unique spectral signatures that can advance estimates of ET and detection of drought stress. This allows analyses based on the physics of spectroscopy and avoids a requirement for continual empirical calibration. These spectral components provide unprecedented information about plant physiological processes, which improve understanding of the regulation of water fluxes and the energy budget. Laboratory, field, and airborne studies of spectral properties in the near‐ and shortwave‐infrared region show strong relationships with plant water relations like water content, relative water content, and water potential. Because water absorption features are spectrally independent of pigment absorptions in the visible region, they provide a new source of information about environmental conditions. These new observations from imaging spectroscopy will lead to better understanding of ecological and hydrological processes. Copyright © 2011 John Wiley & Sons, Ltd.

The effect of rice straw mulch on evapotranspiration, transpiration and soil evaporation of irrigated wheat in Punjab, India

Soil evaporation (Es) is considered to be a non-productive component of evapotranspiration (ET). So, measures which moderate Es may influence the amount of water available for transpiration (T), the productive component of ET. Field experiments investigating the effects of rice straw mulch on components of the water balance of irrigated wheat were conducted during 2006–2007 and 2007–2008 in Punjab, India, on a clay loam soil. Daily Es was measured using mini-lysimeters, and total seasonal ET was estimated as the missing term in the water balance equation. Mulch lowered total Es over the crop growth season by 35 and 40 mm in relatively high and low rainfall years, respectively. Much of this “saved water” was partitioned into T, which increased by 30 and 37 mm in the high and low rainfall years, respectively. As a result, total ET was not affected by mulch in either year. In both years, there was a trend for higher biomass production and grain yield with mulch, but with significant differences only in 2006–2007. Transpiration efficiency (TE) with respect to grain yield was 18.8–19.1 kg ha−1 mm−1 in 2006–2007, and 14.6–16.4 kg ha−1 mm−1 in 2007–2008. While wheat grown in the presence of mulch tended to lower TE, this was only significant in 2007–2008. The results suggest that while mulching of well-irrigated wheat reduces Es, it does not “save” water because the crop compensates by increased T and reduced TE.

Assessing the effect of climate change on mean annual runoff in the Songhua River basin, China

AbstractInfluences of climatic change on the components of global hydrological cycle, including runoff and evapotranspiration are significant in the mid‐ and high‐latitude basins. In this paper, the effect of climatic change on annual runoff is evaluated in a large basin—Songhua River basin which is located in the northeast of China. A method based on Budyko‐type equation is applied to separate the contributions of climatic factors to changes in annual runoff from 1960 to 2008, which are computed by multiplying their partial derivatives by the slopes of trends in climate factors. Furthermore, annual runoff changes are predicted under IPCC SRES A2 and B2 scenarios with projections from five GCMs. The results showed that contribution of annual precipitation to annual runoff change was more significant than that of annual potential evapotranspiration in the Songhua River basin; and the factors contributing to annual potential evapotranspiration change were ranked as temperature, wind speed, vapour pressure, and sunshine duration. In the 2020s, 2050s, and 2080s, changes in annual runoff estimated with the GCM projections exhibited noticeable difference and ranged from − 8·4 to − 16·8 mm a−1 (−5·77 to − 11·53% of mean annual runoff). Copyright © 2011 John Wiley & Sons, Ltd.

산림에서의 젖은 군락 증발 관측: 고찰

산림에서의 차단강수증발(EWC)은 증발산과 강수에 중요한 기여를 한다. 따라서, 산림에서의 수문순환을 이해하기 위해서는 정확한 <TEX>$E_{WC}$</TEX>를 산정하는 것이 중요하다. 본 고찰에서는 <TEX>$E_{WC}$</TEX>의 측정방법을 소개하고, 선행 연구에서 보고된 산림형태(예를 들면, 활엽수림, 침엽수림, 혼효림, 열대림)에 따른 <TEX>$E_{WC}$</TEX> 값과 측정시 고려해야 할 사항에 대하여 논의하였다. 전형적인 <TEX>$E_{WC}$</TEX> 측정에는 물 수지, 에너지 수지 및 Penman-Monteith 방법이 있다. 전반적으로, <TEX>$E_{WC}$</TEX>는 강수량의 5~54%를 차지하였으며, 같은 산림형태내에서도 <TEX>$E_{WC}$</TEX>의 강수량에 대한 기여도는 큰 변동을 보였다. 이러한 변동에는 강수강도, 기상조건, 군락 구조 특성이 영향을 미치는 것으로 나타났다. 따라서 특정 산림형태에서의 <TEX>$E_{WC}$</TEX>의 강수량에 대한 기여도를 정량화하는 것은 어려울 것으로 판단된다. 관측시 발생하는 오차는 <TEX>$E_{WC}$</TEX> 정량화의 불확실성을 증대 시킨다. 물수지 방법의 경우, 풍속의 영향을 받는 강수 관측과 군락 구조의 공간적 비균질성의 영향을 받는 수관통과우 등의 관측 오차를 들 수 있다. 에너지 수지 방법의 경우에는 현열 플럭스와 열저류항의 관측이 주요 오차의 원인이 되며, Penman-Monteith 방법은 공기전도도와 현열의 이류 추정에서 발생하는 오차에 주의를 기울여야 한다. 각 측정방법의 오차를 최소화하고 신뢰할 수 있는 <TEX>$E_{WC}$</TEX>를 얻기위해서는 수문학적 방법과 미기상학적 방법, 즉 물 수지와 에너지 수지 방법을 함께 사용하는 것이 바람직하다. Wet canopy evaporation (<TEX>$E_{WC}$</TEX>) has been recognized as a significant component of total evapotranspiration, especially in forests and therefore it is critical to accurately assess <TEX>$E_{WC}$</TEX> to understand forest hydrological cycle. In this review, I focused on the measurement methods and evaluating the magnitudes of <TEX>$E_{WC}$</TEX> at diverse forest types (e.g., deciduous, coniferous, mixed, and rain forests). I also present the general issues to be considered for <TEX>$E_{WC}$</TEX> measurements. The commonly used measurement methods for <TEX>$E_{WC}$</TEX> include the water balance, energy balance, and the Penman-Monteith (PM) methods. The magnitudes of <TEX>$E_{WC}$</TEX> ranged from 5 to 54% of precipitation based on the literature review, showing a large variation even for a similar forest type possibly related to canopy structure, rainfall intensity, and other meteorological conditions. Therefore, it is difficult to draw a general conclusion on the contribution of <TEX>$E_{WC}$</TEX> to evapotranspiration from a particular forest type. Errors can arise from the measurements of precipitation (due to varying wind effect) and throughfall (due to spatial variability caused by canopy structure) for water balance method, the measurements of sensible heat flux and heat storage for energy balance method, and the estimation of aerodynamic conductance and unaccounted sensible heat advection for the PM method. For a reliable estimation of <TEX>$E_{WC}$</TEX>, the combination of ecohydrological and micrometeorological methods is recommended.

Proposed principles governing how vegetation changes affect transpiration

AbstractUnderstanding global variation in evapotranspiration (ET) is critical for accuracy of climate models, predictions used in water resources management, and assessment of land use change impacts on the water balance of ecosystems—yet we lack unifying principles to predict when transpiration (T) varies with land use. Plant T is a dynamic and often dominant component of ET, and is affected by a variety of processes controlled by land use changes superimposed onto edaphic conditions. We propose the following three principles that determine whether T will vary with changes in vegetation: variation will result if energy balance partitioning has been altered, if deeper or shallower active rooting depth has changed the amount of soil moisture accessible to plants, or if temporary changes in water use add up over longer time scales. Clearly these concepts are not new; however, they are often overlooked in favor of blanket assumptions that large changes in vegetation inevitably alter T. Not so. Our suggested framework incorporates both edaphic and plant traits that determine whether T will vary or not in response to altered land cover conditions. We suggest that this simple set of principles unifies results of wide‐ranging studies of T following land use change and can explain underlying causes of T variation. These principles are illustrated through case studies from four different environments: Pacific Northwest forest, Texas mesquite brushland, Texas savanna and woodland, and Middle Rio Grande riparian forest. The proposed principles seem broadly applicable, should be further evaluated, and are directly relevant for land management. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Evapotranspiration components determined by sap flow and microlysimetry techniques of a vineyard in northwest China: Dynamics and influential factors

► Sap flow and evaporation were measured separately by heat balance sap flow system and micro-lysimeters in our vineyard . Diurnal and seasonal dynamics of sap flow and its environmental controls were analyzed. ► Sap flow rate fluctuated diurnally and seasonally along with solar radiation and vapor pressure deficit. Volumetric soil water content to a depth of 1.0 m (VWC) influenced sap flow rate only when it fell below 0.188 cm 3 cm −3 . However, soil evaporation increased exponentially with VWC. ► When scaling vineyard transpiration from sap flow, leaf area, rather than stem diameter or vine cross-sectional area, should be selected as an indicator of individual sap flow variability. ► The sum of transpiration and evaporation was satisfactorily consistent to evapotranspiration measured by the Bowen ratio energy balance system, demonstrating the applicability of sap flow for measuring grape water use in this region. ► Soil evaporation accounted for 47.3% of total evapotranspiration. Keeping the VWC in such vineyards slightly above ∼60% of the field capacity could reduce the ratio of soil evaporation to evapotranspiration, according to the different response to VWC for sap flow rate and soil evaporation. Large areas of vineyards have been established in recent years in arid region of northwest China, despite limited water resources. Water to support these vineyards is mainly supplied by irrigation. Accurate estimation of vineyard evapotranspiration ( ET ) can provide a scientific basis for developing irrigation management. Transpiration and soil evaporation, as two main components of ET , were measured separately in a vineyard in this region by heat balance sap flow system and micro-lysimeters during the growing season of 2009. Diurnal and seasonal dynamics of sap flow and its environmental controls were analyzed. Daily sap flow rate ( SR l ) increased linearly with solar radiation ( R s ), but showed an exponential increase to its maximum curve as a function of vapor pressure deficit (VPD). Residuals of the two regressions both depended on volumetric soil water content to a depth of 1.0 m (VWC). VWC also significantly influenced SR l . The relationship of them could be expressed by a piecewise regression with the turnover point of VWC = 0.188 cm 3 cm −3 , which was ∼60% of the field capacity. Conversely, soil evaporation ( E s ) increased exponentially with VWC. Thus, we recommended keeping VWC in such vineyards slightly above ∼60% of the field capacity to maintain transpiration while reducing soil evaporation. Vineyard transpiration ( T s ) was scaled from sap flow by using leaf area ( A l ) as it explained 60% of the spatial variability of sap flow. Vine transpiration was 202.0 mm during the period from April 28 to October 5; while that of E s was 181.0 mm. The sum of these two components was very close to ET estimated by the Bowen ratio energy balance method (386.9 mm), demonstrating the applicability of sap flow for measuring grape water use in this region.

Comparison of evapotranspiration estimated by ETWatch with that derived from combined GRACE and measured precipitation data in Hai River Basin, North China

Evapotranspiration (ET) is a fundamental component of the hydrological cycle. It is also the main mode of irrigation and precipitation loss in cultivated lands. Accurate estimation of ET is therefore important for sustainable water resources management, including monitoring droughts and simulating crop production. Whereas precipitation and runoff are measured with reasonable accuracy, ET is difficult to quantify especially at the basin/regional scale. Remote sensing (RS) energy balance models are therefore the commonly preferred methods for estimating ET on large spatial scales. The ETWatch model is one such RS-based model that interactively links several standard RS-algorithms to estimate ET. Another promising alternative method for estimating ET is the terrestrial-based water balance approach. Observations of the Gravity Recovery and Climate Experiment (GRACE) allow sufficient closure of terrestrial water balance. In combination with measured precipitation and runoff data, GRACE data can be used to estimate ET. This paper compares ET estimated by the RS-based ETWatch model with that derived as a residual of the terrestrial-based water balance technique (GRACE plus measured precipitation data) for the approx. 319 000-km2 Hai River Basin (HRB) in North China. About 63 consecutive months of data, spanning December 2002 to February 2008, are used in the study. The amplitudes and phases of the RS-based ETWatch-estimated ET match favourably those of the terrestrial-based water balance technique. The average seasonal (12.7 mm) and annual (16.6 mm) RMSE is less than 10% of the estimated ET. This implies that the RS-based ETWatch model reliably estimates ET at large spatial scales. Time series plots show that the amplitudes of the hydrological components of precipitation, ET and total water storage change are highest in summer and lowest in winter. The GRACE-derived total water storage change shows overall storage loss in the basin. This method could be used not only to fine-tune RS-based ET models, but also to monitor basin/regional water storage change in real-time conditions. Citation Moiwo, J. P., Yang, Y., Yan, N. & Wu, B. (2011) Comparison of evapotranspiration estimated by ETWatch with that derived from combined GRACE and measured precipitation data in Hai River Basin, North China. Hydrol. Sci. J. 56(2), 249–267.

Parameter Optimization for Predicting Soil Water Movement under Crop Residue Cover

Plant residue on the soil surface is considered useful for reducing the evaporation component of crop evapotranspiration. In contrast to soil, the upward movement of water through the plant residue layer is predominantly in the vapor phase rather than the liquid phase. Because of this difference in the mechanism of water movement, physically based numerical models that simulate liquid-dominated water movement in the soil profile have not been tested for water movement through residue. In this article, a new methodology is presented in which the crop residue layer is replaced with a hydraulically equivalent soil layer (ESL), thereby allowing the use of a numerical model, Hydrus-1D, to simulate the changes in soil moisture in a residue-covered production system. Parameter optimization software, PEST, was used to characterize the unsaturated hydraulic function of the ESL. The data for calibrating and validating the model were collected from two different studies in 2010 at the University of California West Side Research and Extension Center in Five Points, California. The simulated soil water content in the profile under the ESL matched the measured soil water content under crop residue cover with RMSE values of 0.004 to 0.013 cm3 cm-3 for calibration and 0.001 to 0.019 cm3 cm-3 for validation. Parameter optimization of the ESL provides a new tool to allow the use of a physically based model such as Hydrus-1D to simulate the many complex interactions between the soil, crop residue, and atmosphere. The methodology may need further validation in different soil, mulch, and climatic conditions. In future research, the effect of crop residue on evaporation for a crop system will be quantified for different growth periods, and efficient irrigation management protocols will be designed to reduce the evaporation component of crop evapotranspiration.

Analyzing Evapotranspiration Components and Crop Coefficients for Catch Crop Field with Small Area at Different Plant Densities in a Greenhouse

We cultivated dent corn as a catch crop for controlling soil salt conditions in field plots at different densities, namely, 7.3 (low), 59.7 (normal) and 119.5 (high) plants m−2 in a greenhouse, where environmental elements and plant growth were measured for analyses of evapotranspiration components relating to the catch crop effects. Using the Penman-Monteith model with specific parameters incorporated for the crop and greenhouse, leaf transpiration and soil evaporation were estimated. Further crop coefficients were analyzed by dividing actual evapotranspiration by a reference evapotranspiration determined from meteorological data. Leaf area index for the normal and high density treatments reached 14 and 22, respectively, which were extremely high values compared to those for various other crops. Transpiration and its ratio to evapotranspiration increased with plant growth and density. Crop coefficients also increased with leaf area index and transpiration, and the coefficients for the normal and high density treatments reached high values of 1.5–2 and 2–3, respectively. These findings reveal characteristic features of plant growth and water dynamics in a catch crop field. The results of this study will contribute to the optimization of catch crop cultivation and to elucidate the crop's effects on soil salt conditions in greenhouses.

Evapotranspiration components and dual crop coefficients of coffee trees during crop production

▶ We determined ET components and Ke and Kcb of coffee trees during production phase. ▶ Regardless irrigation treatment, E represented 35% of ET, while T was 65% of ET. ▶ Recommended Ke is 0.46 for sprinkler irrigation and 0.26 for drip. ▶ Kcb was recommended for both irrigated treatments as a function of daily ETo. Quantifying crop water consumption is essential for many applications in agriculture, such as crop zoning, yield forecast and irrigation management. The objective of this study was to determine evaporation (E), transpiration (T) and dual crop coefficients (Ke and Kcb) of coffee trees during crop production (3rd and 4th year of cultivation), conducted under sprinkler and drip irrigation and no irrigation, in Londrina, Paraná State, Brazil. Crop evapotranspiration (ET) was measured by weighing lysimeters cultivated with plants of cultivar IAPAR 59, E was measured by microlysimeters installed on the lysimeters and T was obtained by the difference between ET and E. The crop coefficient (Kc) was determined for the irrigated treatments as the ratio between ET and the reference evapotranspiration (ETo). Similarly, evaporation coefficient (Ke) and basal crop coefficient (Kcb) were determined as the ratio of E and T, respectively, to the value of ETo, which was estimated by the ASCE Penman–Monteith method on an hourly basis. The values of E and Ke varied due to atmospheric demand and water application method. Those factors, in addition to crop phenology and leaf area evolution, also influenced T and Kcb. Regardless irrigation treatment, the measured values of E represented 35% of ET, while T was 65% of ET. The recommended values of Ke were 0.46 and 0.26 for sprinkler and drip irrigation, respectively. The recommended values of Kcb were 0.52 and 0.82 for sprinkler-irrigated, and 0.5 and 0.65 for drip-irrigated treatments, varying as a function of daily ETo (ETo ≥ or <3 mm day −1 , respectively).

Vertical structure of evapotranspiration at a forest site (a case study)

The components of ecosystem evapotranspiration of a Norway spruce forest ( Picea abies L.) as well as the vertical structure of canopy evapotranspiration were analyzed with a combination of measurements and models for a case study of 5 days in September 2007. Eddy-covariance and sap flux measurements were performed at several heights within the canopy at the FLUXNET site Waldstein–Weidenbrunnen (DE-Bay) in the Fichtelgebirge mountains in Germany. Within and above canopy fluxes were simulated with two stand-scale models, the 1D multilayer model ACASA that includes a third-order turbulence closure and the 3D model STANDFLUX. The soil and understory evapotranspiration captured with the eddy-covariance system in the trunk space constituted 10% of ecosystem evapotranspiration measured with the eddy-covariance system above the canopy. A comparison of transpiration measured with the sap flux technique and inferred from below and above canopy eddy-covariance systems revealed higher estimates from eddy-covariance measurements than for sap flux measurements. The relative influences of possible sources of this mismatch, such as the assumption of negligible contribution of evaporation from intercepted water, and differences between the eddy-covariance flux footprint and the area used for scaling sap flux measurements, were discussed. Ecosystem evapotranspiration as well as canopy transpiration simulated with the two models captured the dynamics of the measurements well, but slightly underestimated eddy-covariance values. Profile measurements and models also gave us the chance to assess in-canopy profiles of canopy evapotranspiration and the contributions of in-canopy layers. For daytime and a coupled or partly coupled canopy, mean simulated profiles of both models agreed well with eddy-covariance measurements, with a similar performance of the ACASA and the STANDFLUX model. Both models underestimated profiles for nighttime and decoupled conditions. During daytime, the upper half of the canopy contributed approximately 80% to canopy evapotranspiration, whereas during nighttime the contribution shifted to lower parts of the canopy.