Total Evapotranspiration Research Articles

Relative Evaporative Losses and Water Balance in Subsurface Drip and Center Pivot–Irrigated Soybean Fields

This research evaluated the relative evaporative losses and water balance components in two soybean [Glycine max (L.) Merr.] fields under subsurface drip irrigation (SDI) and center pivot irrigation (CPI) systems in south-central Nebraska. Meteorological and surface energy balance components, including actual evapotranspiration (ET), above the crop canopy was measured using Bowen ratio energy balance systems installed at the center of both fields. Crop transpiration (T) was estimated based on the variable stomatal resistances using the Penman-Monteith equation in conjunction with fractional green canopy cover. Evaporation (E) losses were estimated as the difference between measured ET and estimated T. Average soil water content (ASWC) in the crop root zone and effective rainfall were estimated using the water balance method. The relative evapotranspiration (ETrel) was 99.8% for the SDI field in both years (2007 and 2008), and it was 103.4% in 2008 and 93.9% in 2010 for the CPI field. The mean ETrel between sites and across years were not significantly different (P>0.05). There were revealing differences in the relative contribution of T and E to total seasonal ET between the SDI and CPI fields. In the SDI field, T and E were 88 and 12% of ET, respectively, in 2007, and in 2008 they were 84 and 16% of ET, respectively. In the CPI field, T and E were 78 and 22% of ET, respectively, in 2008, and in 2010 they were 75 and 25% of ET, respectively. At full canopy cover, T contributed more than 85% of the total ET. Although E is assumed negligible at the full canopy cover stages, the data indicated that E was 6 and 9% of ET in SDI fields during the midseason growth stage at full canopy cover in 2007 and 2008, respectively, and 14 and 21% of ET in CPI fields during the same growth stage in 2008 and 2010, respectively. On a 2-year average, the CPI field had approximately 10% higher E losses as compared with the SDI field under these experimental conditions, which can have a large impact on crop water productivity when crop production under water-limiting conditions is considered. The reduction in E in the SDI field can be expected to be greater in arid and semiarid regions than in subhumid regions.

HYDROGEOLOGICAL SYSTEM ANALYSES OF THE LAKE SIBAYI CATCHMENT, NORTH-EASTERN SOUTH AFRICA

The increased demand for water from Lake Sibayi and the associated groundwater system combined with decreasing rainfall has resulted in a significant reduction in lake levels over the past decade. Moreover, extensive Eucalyptus and pine plantations in the catchment transpire large quantities of groundwater, adding extra pressure on the already stressed water resource. Groundwater recharge was calculated using the chloride mass balance (CMB) method and the results correlated with results obtained from published maps. The methods gave an average recharge estimate of 112.7 mm/a (12% of MAP) and 94.5 mm/a (10% MAP), respectively. The total evaporation and evapotranspiration from the lake and its catchment was estimated at 1 491 mm/a and 1 090 mm/a respectively. A 15-year water balance for the catchment was undertaken to determine the impact of various components on the system. The water balance of Lake Sibayi shows that lake levels fluctuate in response to varying amounts of groundwater and surface water inflow into the lake, seepage loss through the coastal dune, abstraction, and evaporation from the lake. The hydrochemical compositions of the 33 samples collected from various water sources are similar, with the shallow aquifers dominated by Na-Cl and the deep aquifers dominated by a Na-Ca-HCO 3 hydrochemical facies, respectively .

Seasonality of soil water exchange in the soybean growing season in southern Brazil

The processes of water transfer in the soil-plant-atmosphere system are strongly affected by soil use and management. Differences in the dynamics of soil water transfer between no-tillage (NT) and conventional tillage (CT) practices during a soybean (Glycine max) growing season in southern Brazil were assessed in this study. All the water balance components were analyzed during the soybean growing season (2009/2010). Rainfall, runoff, soil water storage and hydro-physical soil properties were analyzed under two tillage systems. The land-atmosphere water vapor exchanges, obtained from eddy covariance stations, were analyzed with regard to the soybean agroecosystem. Characterizations of soil water storage were also formulated in the 2006/2007 and 2008/2009 soybean growing seasons under the NT system. During the periods without rain, the soil water content under NT was greater than under CT. The soil superficial layer, more porous under NT, contributed to less runoff during rainy events. Moreover, under NT conditions the water supply was always high, between 0.2 - 0.5 m. The total evapotranspiration in the soybean agroecosystem growing season was 410.8 mm.

A Study of Shelterbelt Transpiration and Cropland Evapotranspiration in an Irrigated Area in the Middle Reaches of the Heihe River in Northwestern China

The transpiration from shelterbelts and the evapotranspiration (ET) from cropland (maize and vegetables) and orchards (apple) in an irrigated area in the middle reaches of the Heihe River, China, were estimated using a modified Penman-Monteith (P-M) formula and airborne remote sensing data. The results were compared to the shelter transpiration results obtained from measurements of sup flow in tree trunks made with thermal dissipation probes and the latent heat fluxes observed by the eddy covariance technique at flux towers in croplands. The modified P-M formula was found to be an effective means to estimate not only the cropland and orchard ET but also the shelter transpiration. The seasonal variation of shelterbelt transpiration was smaller than those of cropland and orchard ET. Estimates of ET made using the P-M formula along with the remote sensing data showed that 9.9%, 3.1%, and 87.0% of the total ET were allotted to shelterbelts, apple orchards, and cropland, respectively.

Projecting streamflow in the Tangwang River basin (China) using a rainfall generator and two hydrological models

To estimate the impacts of future climate change on streamflow in the Tangwang River basin (TRB) in northeastern China, 2 hydrological models, the Soil and Water Assessment Tool and the Hydro-Informatic Modeling System, were used. These models are driven by future (2021-2050) local rainfall and temperature scenarios downscaled from global climate model (GCM) simulations from the fifth phase of the Coupled Model Intercomparison Project under 2 emission scenarios (Representative Concentration Pathway [RCP] 4.5 and RCP8.5). The downscaling of rainfall is done with the help of a multisite stochastic rainfall generator (MSRG), which extends the 'Richardson type' rainfall generator to a multisite approach using a modified series-independent and spatial-correlated random numbers method by linking its 4 parameters to large-scale circulations using least-squares regressions. An independent validation of the MSRG shows that it successfully preserves the major daily rainfall characteristics for wet and dry seasons. Relative to the reference period (1971-2000), the annual and wet season (April to October) streamflow during the future period (2021-2050) would decrease overall, which indicates that water resources and the potential flood risk would decline in the TRB. The slightly increased dry season (November to March) streamflow would, to some extent, contribute to the 'spring drought' over this region. Although rainfall is projected to remain un changed in the wet season and the whole year, the increased total evapotranspiration due to the increase in temperature would lead to a decline in total streamflow for this basin. The projected streamflow changes from multiple GCMs in this paper could provide a glimpse into a very plausible future for the water resource management community, and would hence provide valuable references for the sustainable management of water and forest ecosystems under a changing climate.

Estimating Mean Long-term Hydrologic Budget Components for Watersheds and Counties: An Application to the Commonwealth of Virginia, USA

Mean long-term hydrologic budget components, such as recharge and base flow, are often difficult to estimate because they can vary substantially in space and time. Mean long-term fluxes were calculated in this study for precipitation, surface runoff, infiltration, total evapotranspiration (ET), riparian ET, recharge, base flow (or groundwater discharge) and net total outflow using long-term estimates of mean ET and precipitation and the assumption that the relative change in storage over that 30-year period is small compared to the total ET or precipitation. Fluxes of these components were first estimated on a number of real-time-gaged watersheds across Virginia. Specific conductance was used to distinguish and separate surface runoff from base flow. Specific-conductance (SC) data were collected every 15 minutes at 75 real-time gages for approximately 18 months between March 2007 and August 2008. Precipitation was estimated for 1971-2000 using PRISM climate data. Precipitation and temperature from the PRISM data were used to develop a regression-based relation to estimate total ET. The proportion of watershed precipitation that becomes surface runoff was related to physiographic province and rock type in a runoff regression equation. A new approach to estimate riparian ET using seasonal SC data gave results consistent with those from other methods. Component flux estimates from the watersheds were transferred to flux estimates for counties and independent cities using the ET and runoff regression equations. Only 48 of the 75 watersheds yielded sufficient data, and data from these 48 were used in the final runoff regression equation. Final results for the study are presented as component flux estimates for all counties and independent cities in Virginia. The method has the potential to be applied in many other states in the U.S. or in other regions or countries of the world where climate and stream flow data are plentiful.

Tropical forest degradation and recovery in fragmented landscapes — Simulating changes in tree community, forest hydrology and carbon balance

Empirical studies on severely fragmented regions suggest that decades after fragmentation, forest edges located near human-modified areas exhibit the structure of early successional states, with lower biomass per area and higher mortality compared to non-edge areas. These habitat changes (edge effects) can also have a considerable impact on ecosystem processes such as carbon and water balance, which in turn have a major impact on human activities.Using field data from a long-term fragmented landscape in the Brazilian Northeastern Atlantic Forest, and the Forest Model FORMIND, we were able to visualize the time scale in which edge effects influence tropical forests by performing 500-year simulations. We observed changes in community composition, aboveground biomass, total evapotranspiration and total runoff.Averages from ten four-hectare simulations show forest biomass degradation lasting around 100 years. If edge effects cease, recovery of biomass lasts around 150 years. Carbon loss is especially intense during the first five years after fragmentation, resulting in a decline of over 5 Mg ha−1y−1 C. Finally, edges of large fragments face an evapotranspiration loss of 43% and total runoff gains of 57% in relation to core areas of large fragments, suggesting that fragmented landscapes can be of significantly lower value in terms of ecosystem services.

Soil mulching can mitigate soil water deficiency impacts on rainfed maize production in semiarid environments

Temporally irregular rainfall distribution and inefficient rainwater management create severe constraints on crop production in rainfed semiarid areas. Gravel and plastic film mulching are effective methods for improving agricultural productivity and water utilization. However, the effects of these mulching practices on soil water supply and plant water use associated with crop yield are not well understood. A 3-yr study was conducted to analyze the occurrence and distribution of dry spells in a semiarid region of Northwest China and to evaluate the effects of non-mulching (CK), gravel mulching (GM) and plastic film mulching (FM) on the soil water supply, plant water use and maize (Zea mays L.) grain yield. Rainfall analysis showed that dry spells of ≥5 days occurred frequently in each of 3 yr, accounting for 59.9–69.2% of the maize growing periods. The >15-d dry spells during the jointing stage would expose maize plants to particularly severe water stress. Compared with the CK treatment, both the GM and FM treatments markedly increased soil water storage during the early growing season. In general, the total evapotranspiration (ET) was not significantly different among the three treatments, but the mulched treatments significantly increased the ratio of pre- to post-silking ET, which was closely associated with yield improvement. As a result, the grain yield significantly increased by 17.1, 70.3 and 16.7% for the GM treatment and by 28.3, 87.6 and 38.2% for the FM treatment in 2010, 2011 and 2012, respectively, compared with the CK treatment. It's concluded that both GM and FM are effective strategies for mitigating the impacts of water deficit and improving maize production in semiarid areas. However, FM is more effective than GM.

Straw Mulching Reduces Maize Yield, Water, and Nitrogen Use in Northeastern China

Straw mulching is used widely to improve soil fertility in the black soil region of northeastern China, but there have been few evaluations of its effects on soil water content, N use by crops, and crop growth. Field experiments were conducted during two growing seasons in Jilin Province, to study the effects of straw mulching on maize yield, evapotranspiration (ET), water use efficiency (WUE), and nitrogen use efficiency (NUE) under rainfed conditions. Measurements were conducted in a treatment with maize straw mulch on the soil surface (SM) and a control treatment with no mulch (NM). The SM treatment caused a significant decline of soil temperature and increase of soil moisture in the early growing season, compared to the NM treatment. However, the use of mulching reduced maize yields by 18% in 2011 and 26% in 2012. It also decreased WUE by 16% in 2011 and 21% in 2012, and decreased NUE by 27% in 2012, though it had no effect on total ET. Therefore, the use of mulch is not recommended as an approach for improving water and N use and maize yields, in the semi‐humid black soil region of northeastern China, when the water content of soil is sufficient for maize growth.

Impact of climate change and watershed interventions on water balance and crop yield in West Seti river sub-basin of Nepal

The Soil and Water Assessment Tool (SWAT) was used to simulate water balances in different cropping patterns under current and future climates in West Seti river Sub-basin, which is located in the far western region of Nepal. The results show that total precipitation over rice, maize, millet, wheat and barley fields were 1002, 818, 788, 186 and 169 mm respectively, whereas total simulated actual evapo transpiration (ET) are 534, 452, 322, 138 and 177 mm respectively under current climate. Actual ET will change by-1.9% in rice, -1.1% in maize, -2.0% in millet, +6.7% in wheat and +5.4% in barley under future climate projections. Results show that yield of maize and millet will decrease by 5.9% and 8.0% whereas yield of rice, wheat and barley will increase by 1.2%, 6.6% and 7.0% respectively. Therefore, the impact of climate change shows that summer crop yields will decrease except of rice and winter crop yields will increase. In general, a result of watersheds interventions shows that the crop yields will increase after the watershed interventions.

Evaluation of CSM-CROPGRO-Cotton for Simulating Effects of Management and Climate Change on Cotton Growth and Evapotranspiration in an Arid Environment

<abstract> <bold><sc>Abstract.</sc></bold> Originally developed for simulating soybean growth and development, the CROPGRO model was recently re-parameterized for cotton. However, further efforts are necessary to evaluate the model’s performance against field measurements for new environments and management options. The objective of this study was to evaluate CSM-CROPGRO-Cotton using data from five cotton experiments conducted at the Maricopa Agricultural Center in Maricopa, Arizona. The field experiments tested ambient atmospheric carbon dioxide (CO₂) versus free-air CO₂ enrichment (FACE) over two growing seasons (1990 and 1991), two irrigation levels and two nitrogen fertilization levels for one growing season (1999), and three planting densities and two nitrogen fertilization levels with optimum irrigation for two growing seasons (2002 and 2003). The model was calibrated by adjusting cultivar and soil parameters for the most optimal or standard treatment of each field trial, and the model’s responses to suboptimal irrigation, suboptimal nitrogen fertilization, nonstandard planting density, and CO₂ enrichment were evaluated. Modifications to the model’s evapotranspiration (ET) routines were required for more realistic ET simulations in the arid conditions of central Arizona because default approaches underestimated seasonal ET up to 157 mm (15% of mean values). Data quality and availability among the field trials were highly variable, but the combination of data sets from multiple field investigations permitted a more thorough model evaluation. Simulations of leaf area index, canopy weight, canopy height, and canopy width responded appropriately compared to measurements from experimental treatments, although some experiments did not impose enough treatment variability to elicit substantial model responses. Simulation results for densely planted cotton were particularly deficient as compared to other experimental treatments. The model simulated seed cotton yield with root mean squared errors ranging from 105 to 1107 kg ha^-1 (3% to 28% of mean values), and total seasonal ET was simulated with root mean squared errors ranging from 12 to 42 mm (1% to 5% of mean values). Seed cotton yield and ET variability due to the imposed experimental treatments were simulated appropriately (p < 0.05), independent of the year-to-year variability due to seasonal factors. Modification of the ET routines permitted maximum simulated crop coefficients ranging from 1.31 to 1.35, which were more realistic than that required for default ET methods in the model. Overall, the evaluation demonstrated appropriate model responses to water deficit, nitrogen deficit, planting density, and CO₂ enrichment. Potential opportunities for further model improvement include the estimation of crop responses to high planting densities, the simulation of cotton maturity and defoliation events, and the calculation of canopy temperature as part of a complete energy balance algorithm.

Tillage practices effect on root distribution and water use efficiency of winter wheat under rain-fed condition in the North China Plain

Water shortage has limited the agricultural sustainable development of North China Plain (NCP), where winter wheat (Triticum aestivum L.) is the major irrigated crop that consumes 60–80% of available deep groundwater for agriculture production, leading to the significant decline in groundwater resource. The protection of water resources is important for the sustainable development of agriculture in NCP. The objective of this study was to evaluate the effect of plow-tillage (PT), rotary-tillage (RT) and no-tillage (NT) on root growth, water consumption characteristics, grain yield, water use and water use efficiency (WUE) under rain-fed condition conducted in a field with 20-year of rotary tillage history. Findings of this research show that plow-tillage (PT) and rotary-tillage (RT) decreased the soil bulk density in the 0–20cm soil depth and the penetration resistance in the 0–30cm soil depth. During two growth seasons, PT had greater root weight density (RWD), root length density (RLD) and root surface density (RSD) than those under NT across the 0–110cm soil profile at the tillering stage and in the 0–40cm soil profile at the flowering stage, respectively. However, RWD, RLD and RSD of PT were lower than NT at 0–10cm soil depth and greater at 10–20cm soil depth at the ripening stage. Similar trends were observed under RT compared with NT. Soil water content (SWC) under PT and RT were lower compared with NT from tillering to flowering stage across 0–110cm, but higher than under NT in 0–20cm soil profile at ripening stage. Evapotranspiration (ET) values under PT were higher than under NT from sowing to flowering stages, but significantly lower at the ripening stage. Moreover, tillage practices had no notable influences on pre-planting soil water storage and total ET under rain-fed condition during two growing season, but PT significantly enhanced grain yield through higher spike number and grain weight compared with NT, which led to higher WUE under PT. The findings of this study show that PT practice can reduce soil bulk density and penetration resistance at the tillage zone, which can lead to greater RWD, RLD and RSD and greater ET from tillering to flowering stage. This can increase plant population and cause greater WUE and grain yield under rain-fed condition.

Evapotranspiration of bush encroachments on a temperate mire meadow – A nonlinear function of landscape composition and groundwater flow

As widely recognized, bush encroachment on mire meadows induces species and habitat biodiversity. However, it is unclear if expansion of either a coherent, continuous forest or randomly distributed groups of trees, so-called tree isles, in formerly open landscapes of mire meadows influences differently the water balance of the system due to changing vegetation structure, landscape, and thus evapotranspiration. In this paper we use a quasi-3D unsaturated–saturated groundwater flow model to reveal the feedback between the actual evapotranspiration and diurnal phreatic groundwater level dynamics. A schematized numerical experiment was setup to elucidate the ecohydrological functioning of mire meadows in the Biebrza Valley (NE Poland), which have been influenced by bush encroachment due to abandonment of mowing. Model calibration and validation was based on field collected data on groundwater dynamics showing the feedback between the evapotranspiration and diurnal groundwater level fluctuations. The model was applied for different landscape compositions of encroachments as well as different stages of shrubby vegetation expansion in a formerly open meadow. It is concluded that for coupled unsaturated–saturated flow models of mires, analysis of diurnal groundwater level fluctuations combined with evapotranspiration quantification is an efficient yet simple method for model calibration and validation. Results of the modelling experiment indicated that the tree-isle-type of bush encroachment entails higher losses of water from the system due to evapotranspiration than coherent forest expansion. For a hypothetical summer drought and an encroachment cover of 50% the total evapotranspiration is shown to be 13% higher for the tree-isle-type encroachment than for expansion of a coherent forest. Consequently, it is concluded that conservation of mires requires continuous control of encroachment not only because of potential loss of biodiversity, but more importantly to limit significant loss of water due to increased evapotranspiration.

Stable oxygen isotope and flux partitioning demonstrates understory of an oak savanna contributes up to half of ecosystem carbon and water exchange.

Semi-arid ecosystems contribute about 40% to global net primary production (GPP) even though water is a major factor limiting carbon uptake. Evapotranspiration (ET) accounts for up to 95% of the water loss and in addition, vegetation can also mitigate drought effects by altering soil water distribution. Hence, partitioning of carbon and water fluxes between the soil and vegetation components is crucial to gain mechanistic understanding of vegetation effects on carbon and water cycling. However, the possible impact of herbaceous vegetation in savanna type ecosystems is often overlooked. Therefore, we aimed at quantifying understory vegetation effects on the water balance and productivity of a Mediterranean oak savanna. ET and net ecosystem CO2 exchange (NEE) were partitioned based on flux and stable oxygen isotope measurements and also rain infiltration was estimated. The understory vegetation contributed importantly to total ecosystem ET and GPP with a maximum of 43 and 51%, respectively. It reached water-use efficiencies (WUE; ratio of carbon gain by water loss) similar to cork-oak trees. The understory vegetation inhibited soil evaporation (E) and, although E was large during wet periods, it did not diminish WUE during water-limited times. The understory strongly increased soil water infiltration, specifically following major rain events. At the same time, the understory itself was vulnerable to drought, which led to an earlier senescence of the understory growing under trees as compared to open areas, due to competition for water. Thus, beneficial understory effects are dominant and contribute to the resilience of this ecosystem. At the same time the vulnerability of the understory to drought suggests that future climate change scenarios for the Mediterranean basin threaten understory development. This in turn will very likely diminish beneficial understory effects like infiltration and ground water recharge and therefore ecosystem resilience to drought.

Partitioning evapotranspiration in an intact forested watershed in southern China

AbstractAccurate estimates and partition of evapotranspiration (ET) are difficult but critical for understanding terrestrial ecosystem processes and primary productivity. A multi‐year, multi‐technique study including the catchment water balance (12 years), a semi‐empirical ET model (9 years), eddy covariance (9 years), canopy water balance (1 year) and sap flow (1 year) techniques was conducted to measure ET and its components within an intact forested watershed in the subtropical region of southern China. Over a 9‐year period, the estimates of annual ET using the catchment water balance, eddy covariance and semi‐empirical ET model techniques were in close agreement, averaging 809.9 ± 62.8, 803.8 ± 36.6 and 801.6 ± 46.9 mm per year, respectively, amounting to an average of approximately 50.2% of the mean annual precipitation. Qualitative similarities in seasonal and diurnal variation were observed between the sap flow and eddy covariance estimates of water flux. Sap flow estimates of transpiration were approximately 60.2% of the annual ET estimated with the eddy covariance technique. Interception evaporation was 31.7% of the total ET and was demonstrated to be a main contributor of total evaporation. Soil evaporation was calculated to be approximately 8.1% of the total annual ET and 4.7% of the annual precipitation. Furthermore, daytime transpiration exhibited a logarithmic increase with increases in the vapour pressure deficit (VPD) on daily scale in both the wet and dry seasons, and tended to level off when the VPD was &gt;1 kPa because of the stomatal regulation of transpiration. Correspondingly, nighttime sap flow amounted to an average of approximately 6% of the total daily sap flow, and no significant correlations with the VPD, air temperature or relative humidity were found. Essential differences among the mechanisms of evaporation and transpiration were demonstrated, suggesting that soil evaporation is primarily a climate‐driven process, with air temperature and wind speed as the predominant driving forces. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Post‐ranching tree–grass interactions in secondary Acacia zanzibarica woodlands in coastal Tanzania – an experimental study

AbstractQuestionsWe studied a humid savanna rangeland, abandoned in 2000, where intensive cattle grazing had led to widespread encroachment by Acacia zanzibarica. We asked whether the acacia trees were able to regenerate in the absence of domestic livestock, either beneath acacia canopies or in artificial clearings.LocationTropical coastal Tanzania (former Mkwaja Ranch, now in Saadani National Park).MethodsWe set out a total of 48 plots on four sites in November 2001, and assigned them to three treatments: trees felled (FN), trees felled and the stumps poisoned (FP) with Triclopyr, and no intervention (controls, NN). We analysed soils of plots for texture and nutrients. In two wet (July 2002 and 2003) and one dry (February 2003) seasons we assessed grass and tree leaf biomass and transpiration rates, and counted acacia seedlings and resprouts. The effects of treatments (controlled for site and other co‐variables) on grass growth and acacia recruitment were determined statistically using general linear models (GLM).ResultsAcacia leaves had a much higher stomatal conductance than grasses, with the consequence that total evapotranspiration in woodland was higher than in clearings. In the wet seasons, grass biomass and seedling densities were significantly higher in clearings than in control plots, which we attributed to more favourable moisture conditions. In the dry season, by contrast, we found no differences, and all seedlings had died. On FN plots, 71% of stumps, and on FP plots, 11% resprouted (coppicing), but only a quarter of these shoots survived until July 2003. Root suckering occurred spontaneously at low densities. No root suckers or resprouts grew beyond the grass layer.ConclusionsAcacia woodlands do not regenerate in the absence of cattle grazing, and tree cutting – in combination with appropriate fire management – could potentially accelerate re‐establishment of open grassland. However, regeneration might occur in the future due to the increasing wildlife populations within the new national park.

Development of a remotely sensing seasonal vegetation‐based Palmer Drought Severity Index and its application of global drought monitoring over 1982–2011

AbstractVegetation effects are currently disregarded in Palmer Drought Severity Index (PDSI), and the sensitivity of PDSI to the choice of potential evaporation (EP) parameterization is often a concern. We developed a revised self‐calibrating PDSI model that replaces EP with leaf area index‐based total evapotranspiration (ARTS E0). It also included a simple snowmelt module. Using a unique satellite leaf area index data set and climate data, we calculated and compared ARTS E0, three other types of EP (i.e., Thornthwaite EP_Th, Allen EP_Al, and Penman‐Monteith EP_PM), and corresponding PDSI values (i.e., PDSI_ARTS, PDSI_Th, PDSI_Al, and PDSI_PM) for the period 1982–2011. The results of PDSI_ARTS, PDSI_Al, and PDSI_PM show that global land became wetter mainly due to increased precipitation and El Niño–Southern Oscillation (ENSO) effect for the period, which confirms the ongoing intensification of global hydrologic cycle with global temperature increase. However, only PDSI_Th gave a trend of global drying, which confirms that PDSI_Th overestimates the global drying in response to global warming; i.e., PDSI values are sensitive to the parameterizations for Ep. Thus, ARTS E0, EP_Al, and EP_PM are preferred to EP_Th in global drought monitoring. In short, global warming affects global drought condition in two opposite ways. One is to contribute to the increases of EP and hence drought; the other is to increase global precipitation that contributes to global wetting. These results suggest that precipitation trend and its interaction with global warming and ENSO should be given much attention to correctly quantify past and future trends of drought.

Transpiration and canopy conductance of two contrasting forest types in the Lesser Himalaya of Central Nepal

As part of a broader investigation of the hydrological implications of reforesting degraded pastures in the Middle Mountain Zone of Central Nepal, tree transpiration (Et) and canopy conductance (gc) of a natural broad-leaved forest and a mature planted pine forest were quantified using sap flow measurements and concurrent climatic and soil water observations. Estimated annual Et totals were subsequently combined with the corresponding interception losses (Ei) to calculate annual evapotranspiration totals (ET) for the two forests. Calculated Et was strongly dependent on atmospheric vapour pressure deficit (VPD) but much less on short-wave radiation (Rs) while there was little evidence of any limitation by soil water deficits despite a strongly seasonal rainfall regime. Both forests transpired readily throughout the dry season, except during the period of maximum leaf fall (March–April). Annual Et by the tree stratum amounted to 163mm and 280mm in the natural and planted forest, respectively, representing 12.2% and 19.6% of the corresponding incident rainfall totals. Estimated annual ET (including understory and litter evaporation) values were 524mm and 577mm, respectively (39.3% and 40.5% of rainfall). Maximum daily estimates of canopy conductance gc ranged from 4.85mms−1 in the natural forest to 11.46mms−1 in the planted forest and averaged 1.69 (±0.74) and 3.28 (±1.55)mms−1, respectively. A strong response of gc to changes in VPD was detected in both forests. Combined, the higher ET of the planted forest (particularly during the dry season), plus the heavy usage of the forest by the local population and its correspondingly poor soil hydrological functioning (Ghimire et al., 2013b), are considered to be important factors in the observed decline in dry season streamflow volumes following large-scale reforestation of degraded pasture- and scrubland with pines in the region.

Evapotranspiration and energy balance dynamics of a semi-arid mountainous steppe and shrubland site in Northern Mongolia

The semi-arid forest-steppe ecotone in the Khentii Mountains in Northern Mongolia is characterized by southerly exposed steppe slopes, shrublands in the floodplain of river valleys and northerly exposed taiga slopes. Here, the freshwater for the downstreaming agricultural, rural and mining sectors is generated. To gain insight into the main water-generating sources in mountainous river basins, we studied the actual evapotranspiration (ET) and its main influencing eco-hydrometeorological parameters at a steppe and a shrubland site during the growing season 2012. Latent (LE) and sensible (H) heat fluxes were measured using the Bowen ratio energy balance method, and the surface soil heat flux (G 0) was determined with the PlateCal and GradCal approach. During water-stressed soil conditions at the steppe site, volumetric water content (VWC) values of 0.05 m3 m−3 (−10 cm) resulted in significant limitations of the mean actual ET rates to 0.4 mm day−1. In contrast, high VWC with maxima of >0.4 m3 m−3 and high mean actual ET rates >6.0 mm day−1 were observed during water-unstressed soil conditions. As precipitation and change of stored soil moisture were approximately equal to actual ET, the contribution of freshwater from mountainous steppe within the forest-steppe ecotone can be neglected. At the shrubland site, which is underlain by a shallow aquifer system, VWC remained relatively constant with mean values of 0.4 m3 m−3 (−10 cm). A multiple regression analysis indicated that actual ET, with mean rates of 2.3 mm day−1, was not significantly limited by precipitation and VWC, but by net radiation (R n) and air temperature. As precipitation exceeded total actual ET, shrub covered river floodplains can contribute to freshwater generation.

Prediction of cotton yield and water demand under climate change and future adaptation measures

Cotton is the main cultivated cash crop in Northwest China. This study is motivated by the challenge of economically growing cotton in the face of climate change, which could lead to changes in cotton variety, phenology, water demand, heat requirement and yield. The APSIM-OzCot crop growth model is used to simulate cotton cultivation at two sites with different climatic conditions (Shihezi and Alaer, Xinjiang Province). The model is fully calibrated and validated using observations. Simulations forced with future climate data downscaled from the HadCM3 Global Climate Model show that the response of cotton phenology, yield and water use to climate change is different for different cultivation, sites, greenhouse gas emission scenarios and time horizons. Under the SRES A1B and B1 emissions scenarios, cotton yield and water use are greater in the future than in the 1961–1990 period while the growing season is shorter. Under the SRES A2 emissions scenario, however, yields at cold sites drop after 2070 due mainly to the shortening of cotton growth periods. Thus, in the cold regions, varieties with short growth periods are replaced by those with long growth periods. The results show that, compared with current local varieties, cotton yields increase by 356kg/ha with the medium maturity variety “K7” and 473kg/ha with the late maturity variety “ZM49” under the A2 scenario by 2070. Total evapotranspiration correspondingly increases by 69mm (“K7”) and 92mm (“ZM49”). However, water use efficiency increases by 0.32kg/m3 to 0.34kg/m3 (6.3%) and 0.35kg/m3 (9.4%) for “K7” and “ZM49”, respectively. A reasonable adaptive strategy to maintain cotton yields in the future may be to decrease the area over which cotton is planted and raise water use efficiency.