Reduction In Evapotranspiration Research Articles

Transpiration in the water-limited regime: soil-plant-atmosphere interactions

The use of vegetation to improve stability of natural and engineered slopes is an engineering Nature Based Solution. One effect of vegetation is to reinforce slopes ‘hydrologically’, i.e., by generatingsuction by the removal of soil water via transpiration. In turn, the depletion of soil water content reduces the hydraulic conductivity of the shallow layers of the soil, and this hinders rainwater infiltration during the wet period, possibly preserving suction in the deeper layers susceptible to failure. To improve upon thisstabilising technique, it is key to develop transpiration models that account for the hydraulic characteristics of the soil and plant (below- and above-ground). In this way, modelling can guide the choice of the plant functional traits. This paper first discusses the conceptual and experimental limitations of common empirical evapotranspiration reduction functions (e.g. Feddes function) and then revisits the physically-based ‘bottlenecks’ generating the decline in evapotranspiration in the water-limited regime within the framework of the Soil-Plant-Atmosphere Continuum.

Changes in land use and cover and their consequences on the spatial distribution of evapotranspiration and climatic elements in southwest Piauí, MATOPIBA

With the advance of agriculture in Brazil, the occurrence of environmental disturbances that can reach different levels, such as the change of use and land cover, is perceived. In the southwest of Piauí, it is noticeable the large portion of Cerrado that has been replaced by crops and pastures, resulting in several impacts, such as changes in evapotranspiration. In the literature there are several methods to estimate it, among them the remote sensing techniques stand out. Therefore, the objective of this work was to quantify agricultural expansion and understand its consequences on real evapotranspiration and on the configuration of meteorological elements in the municipality of Baixa Grande do Ribeiro, through the products of the United States Geological Survey (USGS) accessed through Google Earth. Engine. Platform (GEE), between 2000 and 2020. The real evapotranspiration (ETa) and leaf area index (LAI) data come from the MODIS sensor products. The meteorological data were obtained from the website of the National Aeronautics and Space Administration Prediction Of Worldwide Energy Resources (NASA POWER). The maps were made in QGIS 3.18.1 software from data processed on the GEE platform. The evolution of forest and agricultural areas and images of land use and cover were obtained using the MapBiomas platform. It was observed that between 2000 and 2020, about 27% of the area of native vegetation was replaced, and soybean cultivation grew by approximately 2618%. In addition, the removal of native vegetation for the establishment of pastures and agricultural crops has contributed to the reduction of evapotranspiration and the leaf area index, in addition, it may be associated with the increase in surface and air temperatures and reduction of the relative humidity of the air. This implies the need to create and implement public policies aimed at controlling deforestation in the region, as well as greater inspection in the sphere of environmental crimes.

Crop response to thermal stress without yield loss in irrigated maize and soybean in Nebraska

Thermal sensing provides rapid and accurate estimation of crop water stress through canopy temperature data. Canopy temperature is highly dependent on the transpiration rate of the leaves. It is usually assumed that any reduction in crop evapotranspiration (ET) leads to crop yield loss. As a result, an increase in canopy temperature due to a decrease in crop ET would indicate crop yield loss. This research evaluated the hypothesis that crop water stress could be detected using canopy temperature measurements (increased leaf temperature) from infrared thermometers (IRTs) before incurring crop yield loss. This would be possible in a narrow range when the photosynthesis rate (and carbon assimilation) is limited by solar radiation (energy-limiting water stress) while the leaf has abundant carbon dioxide for photosynthesis. Once photosynthesis becomes limited by carbon dioxide (carbon-dioxide-limiting water stress), then yield reduction would occur. In this field experiment, measured response variables included the integrated crop water stress index (iCWSI), ET, and crop yield for maize and soybean during the 2020 and 2021 growing seasons. The irrigation was applied at four different refill levels: rainfed (0%), deficit (50%), full (100%), and over (150%). The irrigation depth was prescribed using four different irrigation methods. The field was irrigated with a center pivot irrigation system, which was also used as a platform to mount IRT sensors. The iCWSI thresholds required for irrigation management were determined using the iCWSI dataset collected in 2020. The low, medium, and high iCWSI thresholds were 120, 150, and 180, respectively for maize and 110, 130, and 150, respectively for soybean. These thresholds should be updated with iCWSI data from future studies in this region to increase the credibility of the thresholds for irrigation management. The mean iCWSI values for consecutive days after a wetting event substantially increased with time for each irrigation level and a larger range in iCWSI values was observed among the irrigation levels after three days from a wetting event. The seasonal iCWSI for different levels were found to be negatively correlated with seasonal evapotranspiration for both years. The correlations between seasonal ET and crop yield were significant with the rainfed and deficit levels for maize (p-value < 0.001) and soybean (p-value = 0.04) in 2020. The iCWSI and yield data for the fully watered plots indicated that thermal stress was detected using the sensing system without incurring yield loss (i.e., energy-limiting water stress). The ET and yield data for 2021 indicated that reduction in seasonal crop ET did not result in yield loss which also supported the hypothesis. Future studies should investigate whether this phenomenon of detecting crop water stress in an early stage without yield loss is observed in other climates and locations.

Urbanization altered atmospheric humidity diurnally and seasonally through ecohydrological processes in five urban agglomerations in China

The large-scale conversion of vegetated land to urban use leads to a significant reduction in evapotranspiration (ET) due to the lack of vegetation, which may aggravate urban dry island (UDI) effect. Analysis of diurnal and seasonal (e.g. growing season) variations in UDI can help us to better identify the role of ET in processes of UDI. We compared six-hourly weather observation data (1980–2017) from 140 paired urban-rural stations across a large climatic gradient in China to explore how near-surface atmospheric dryness changed both diurnally and seasonally, and its relationship with urbanization-associated ecohydrological processes. We showed that the difference in atmospheric dryness (i.e. UDI intensity) between urban and rural areas, as measured by specific humidity (Δq < 0), is more pronounced during the daytime and growing seasons. The nighttime urban wet island (UWI, Δq > 0) effect partially offset daily UDI effect, which has made the latter underestimated. Intensified nighttime urban heat island (UHI) reduced the diurnal temperature range (DTR) in cities and thus enhanced nighttime UWI effect from 2000 to 2010. However, after 2010, nighttime UWI effect weakened or disappeared, whereas nighttime UDI intensified in humid cities, resulting in a significant increase in daily UDI. Intensified UHI and UDI are often closely coupled (synchronous occurrence) through latent heat (LE) or ET processes, especially after 2010. Our results indicate that the conversion of vegetated lands to urban impervious surface, especially in humid regions, leads to the reduction in transportation during daytime and evaporation at night in urban cores, which alters the relationships between near-surface air temperature, atmospheric moisture, and ET. The present diurnal and seasonal variations in UDI were delineated in detail to explicate the patterns and interconnections of local urban climate and surface ecohydrological processes, which are critical for ecosystem services in urban landscape design.

Amplified warming from physiological responses to carbon dioxide reduces the potential of vegetation for climate change mitigation

Global warming is increasing due to the ongoing rise in atmospheric greenhouse gases, and has the potential to threaten humans and ecosystems severely. Carbon dioxide, the primary rising greenhouse gas, also enhances vegetation carbon uptake, partially offsetting emissions. The vegetation physiological response to rising carbon dioxide, through partial stomatal closure and leaf area increase, can also amplify global warming, yet this is rarely accounted for in climate mitigation assessments. Using six Earth System Models, we show that vegetation physiological response consistently amplifies warming as carbon dioxide rises, primarily due to stomatal closure-induced evapotranspiration reductions. Importantly, such warming partially offsets cooling through enhanced carbon storage. We also find a stronger warming with higher leaf area and less warming with lower leaf area. Our study shows that the vegetation physiological response to elevated carbon dioxide influences local climate, which may reduce the extent of expected climate benefits offered by terrestrial ecosystems.

Yield and water use relationships of potato under different timing and severity of water stress

Water scarcity is a prevailing problem that constrains crop production in many regions worldwide. The application of deficit irrigation strategy is a reliable approach, which may contribute to some degree to overcome the problem of global water scarcity. A two-year study was conducted to assess the effects of timing and severity of the deficit irrigation on yield, water productivity (WP) and yield response factor (Ky) of potato. In 2018, the treatments included: 100% (V100) of crop evapotranspiration (ET), 50% (V50–100) reduction of ET until end of tuber initiation, 50% (V100–50) reduction of ET during the middle of tuber bulking up to maturity stage and no irrigation following plant establishment (V0). In 2019, the treatments included: V100 and V0 as in the previous year, deficit irrigation at 50% of ET (DI50), partial root-zone drying (PRD) at 75 or 50% of ET applied during the whole growing season. Deficit irrigation at a reduced rate of 50% of ET crop during the first part of the crop cycle (V50–100) strongly depressed total yield while limiting irrigation water for the last part of the crop cycle (V100–50) did not induce significant losses in yield. Deficit irrigation positively affected water productivity, especially with V100–50, suggesting that water efficiency was closely related to the growth stage than to the available water in the soil. In 2019, reducing water to 75% of V100 with PRD75 did not significantly affect tuber yield, while the further reduction in the irrigation water with PRD50 or DI50 resulted in a substantial yield decrease. Although the same amount of water was applied to both treatments, the reduction in yield was 41% with DI50 compared to 28% with PRD50, where the latter proved to be superior in maintaining yield. However, an appreciable increment in WP was observed with both PRD treatments unless the water stress is too high to depress the yield. The sensitivity of potato to water stress was more remarkable when water was applied at individual growth stages than at different severities. In contrast, potatoes have been tolerant to both stress timing and severity in total dry matter. These results showed that using deficit irrigation late of the season or with PRD can improve water-saving, minimize yield losses, and consider optimal strategies for drip-irrigated potatoes in the arid regions.

Modeling the role of lateral surface flow in low‐relief polygonal tundra

AbstractIce‐wedge polygon troughs play an important role in controlling the hydrology of low‐relief polygonal tundra regions. Lateral surface flow is confined to troughs only, but it is often neglected in model projections of permafrost thermal hydrology. Recent field and modeling studies have shown that, after rain events, increases in trough water levels are significantly more than the observed precipitation, highlighting the role of lateral surface flow in the polygonal tundra. Therefore, understanding how trough lateral surface flow can influence polygonal tundra thermal hydrology is important, especially under projected changes in temperatures and rainfall in the Arctic regions. Using an integrated cryohydrology model, this study presents plot‐scale end‐of‐century projections of ice‐wedge polygon water budget components and active layer thickness with and without trough lateral surface flow under the Representative Concentration Pathway 8.5 scenario. Trough lateral surface flow is incorporated through a newly developed empirical model, evaluated against field measurements. The numerical scenario that includes trough lateral surface flow simulates discharge (outflow from a polygon) and recharge (rain‐induced inflow to a polygon trough from upslope areas), while the scenario that does not include trough lateral surface flow ignores recharge. The results show considerable reduction (about 100–150%) in evapotranspiration and discharge in rainy years in the scenarios ignoring trough lateral surface flow, but less effect on soil water storage, in comparison with the scenario with trough lateral surface flow. In addition, the results demonstrate long‐term changes (~10–15 cm increase) in active layer thickness when trough lateral surface flow is modeled. This study highlights the importance of including lateral surface flow processes to better understand the long‐term thermal and hydrological changes in low‐relief polygonal tundra regions under a changing climate.

Vegetation Physiological Response to Increasing Atmospheric CO2 Slows the Decreases in the Seasonal Amplitude of Temperature

AbstractThe seasonal amplitude of temperature (SAT) decreases due to non‐uniform seasonal warming, which exerts critical impacts on ecosystem functions and services. Previous studies attributed the SAT decrease to changed seasonal budget of atmospheric radiative forcing (RAD), however, potential impacts of vegetation physiological response to CO2 (PHY) on SAT are largely underexplored. Here, we examine the role of PHY in modulating SAT using Earth system model simulations. Globally, PHY continuously increases SAT, offsetting about 55% of the RAD‐driven SAT reductions under quadrupled CO2. The PHY‐induced SAT enhancement is particularly pronounced in the northern mid‐to‐high latitudes. Such enhancement is driven directly by stomatal closure‐induced evapotranspiration reduction that amplifies warming, and indirectly by suppressing cloud formation that increases downward shortwave radiation. Our results highlight the critical role of vegetation in mediating temperature seasonality and the necessity for a full understanding of PHY feedback when evaluating the benefits of terrestrial ecosystems in mitigating climate change.

Impact of Forest Conversion to Agriculture on Hydrologic Regime in the Large Basin in Vietnam

Deforestation due to agricultural land expansion occurred greatly during 1994 to 2005 with a high proportion of forests being converted into agriculture in the upstream Dong Nai river basin in Vietnam. Most of these conversions included expansions of coffee plantations in Dak Lak and Lam Dong provinces, which are in the world’s Robusta coffee production area. The aim of this study is to quantify the impact on the water cycle due to the conversion of forest to coffee plantations in a tropical humid climate region by the application of a hydrological model: soil and water assessment tool (SWAT). The model was calibrated with climate data from 1980–1994, validated with climate data from 1995–2010, and verified with statistical indicators such as Nash–Sutcliffe efficiency (NSE), percent bias (PBIAS), and ratio of the root mean square error (RSR). The simulations indicated that forest conversions into agriculture (expansion of coffee plantations) had significantly increased surface runoff (SUR) while actual evapotranspiration (ET), soil water content (SW), and groundwater discharge (GW) decreased. These changes are mainly related to the decrease in infiltration and leaf area index (LAI) post land cover changes. However, the soil was not thoroughly destroyed after deforestation due to the replacement of the lost forest with crops and vegetation. Therefore, changes in infiltration were marginal and not sufficient to bring large changes in the annual flow. Higher reductions in ET and SW were proposed, resulting in reduced streamflow in the dry season at the basin where the proportion of agricultural land was higher than the forest cover. Besides the plantation expansion, which resulted in streamflow reductions in the dry season, an existing problem was over-irrigation of coffee plantations that could likely deplete groundwater resources. Hence, balancing economic benefits by coffee production and mitigating groundwater depletion issues should be prioritized for land use management in the study area.

Nature-based solutions in mountain catchments reduce impact of anthropogenic climate change on drought streamflow

Quantifying how well Nature-based Solutions can offset anthropogenic climate change impacts is important for adaptation planning, but has rarely been done. Here we show that a widely-applied Nature-based Solution in South Africa – invasive alien tree clearing – reduces the impact of anthropogenic climate change on drought streamflow. Using a multi-model joint-attribution of climate and landscape-vegetation states during the 2015–2017 Cape Town “Day Zero” drought, we find that anthropogenic climate change reduced streamflow by 12–29% relative to a counterfactual world with anthropogenic emissions removed. This impact on streamflow was larger than corresponding reductions in rainfall (7–15%) and reference evapotranspiration (1.7–2%). Clearing invasive alien trees could have ameliorated streamflow reductions by 3–16% points for moderate invasions levels. Preventing further invasive alien tree spread avoided potential additional reductions of 10–27% points. Total clearing could not have offset the anthropogenic climate change impact completely. Invasive alien tree clearing is an important form of catchment restoration for managing changing hydroclimatic risk, but will need to be combined with other adaptation options as climate change accelerates.

Seasonal flashiness and high frequency discharge events in headwater streams in the North Carolina Piedmont (United States)

AbstractFirst‐order streams are rarely the focus of research even though they play an important hydrological and ecological role in upland landscapes and may be more susceptible to human impacts than larger streams. We studied 16 first‐order streams in the North Carolina Piedmont to examine relationships between land use, discharge, flashiness and conductivity in the headwaters of the Rocky River and McDowell Creek. Stream gauges were installed on each of the 16 headwater streams to provide constant monitoring of stream depth, temperature and specific conductance. Rating curves were created for each site using models that combined field measured and Manning's calculated discharge. Initial attempts to measure flashiness using the Richards‐Baker Index (RBI) indicated that it is not a good proxy for flashiness in small streams because its use of median daily flows do not capture fluctuations that occur over short time periods. Instead, we developed a new method that focused on the number of days each stream flows above its 99th percentile flow (DMQ99). We also examined individual storm events across multiple seasons. We found that headwater streams in the Piedmont are flashier than more commonly gauged second and third order streams because first‐order streams are more directly connected to the landscape. Streams located in suburban areas tend to be flashier than streams in forested areas as a result of the increase in impervious surfaces and the reduction of evapotranspiration (ET) caused by development. Additionally, we found that all streams become flashier in the winter when ET is lower, suggesting important feedbacks between physical and biological systems. All streams experience periodic extreme conductivity events, indicating significantly impaired ecological conditions. More developed basins generally have higher maximum specific conductance compared with less developed basins although streams have significant natural variability as well. Overall, our results indicate that first‐order streams in the Piedmont are highly impacted by development and that extra caution should be exercised when altering landscapes near small streams.

Deforestation triggering irreversible transition in Amazon hydrological cycle

The Amazon is hypothesized to reach an irreversible ‘tipping point’ when deforestation slows the hydrological cycle sufficiently that tropical forest ecosystems cannot be sustained. However, inception of such a tipping point has not been supported by observations and the relevant links between deforestation and atmospheric moisture recycling are poorly understood. Here we show that reduction in evapotranspiration from 20 years of deforestation dried the atmosphere persistently and caused moisture decoupling, i.e. an opposite sign of moisture change between the lower and middle troposphere. Increased deforestation exacerbated the lower troposphere drying and caused it to penetrate deeper into the middle troposphere in the dry and transition seasons over monsoon forests and savannas. Deforestation induced warming-enhanced buoyant updrafts, elevated hot and dry air and thereby reduced downward mixing of water supplies from the tropical Atlantic that normally moisten the Amazon forests. The severe atmospheric desiccation in the southern and eastern Amazon cannot be compensated by enhanced water supplies from the Atlantic Ocean, demonstrating an irreversible transition in Amazon hydrological cycle exacerbated by rapid deforestation. The more recent drying through the seasons over rainforests and during the wet season over the transition zones from rainforests to monsoon forests and savannas, however, suggests a window of opportunity for preventing ecosystem collapse with forest conservation.

Reduction of Evapotranspiration in Microenvironment Conditions of Table Grape Vineyards Protected by Different Types of Plastic Covers

Saving water is a major challenge to increase environmental sustainability, particularly in semi-arid regions where most table grapes are produced. Water use is driven by atmospheric demand, which combines effects of solar radiation (prominent factor), wind, air temperature and humidity. Covering table grapes with transparent plastics is spread in many regions. Covers lower incoming solar radiation and wind speed, changing air temperature and humidity. This study assessed the effects of two plastic covers on reference evapotranspiration (ET0) in comparison to the open field. For two years, two vineyards (cv. Victoria) trained to overhead trellis systems (tendone) were covered with two transparent polyethylene sheets: an agrotextile fabric (C), and a commercial film (S). The sheet spectrophotometric properties were analyzed and the radiometric coefficients calculated. Micrometeorological data were recorded in the covered vineyards and in a nearby uncovered one. ET0 was calculated for June and July using the simplified Penman–Monteith equation. The coefficient of transmissivity to total photosynthetic radiation, a proxy of net radiation, was 73% for cover C and 83% for cover S. On average, ET0 decreased by 35% under cover C and 31% under cover S. Hence, in addition to providing protection from external agents, covers represent a valid tool for saving water in table grape viticulture; nevertheless, their radiometric properties should be considered and optimized to better achieve this goal.

Attributions of Evapotranspiration and Gross Primary Production Changes in Semi-Arid Region: A Case Study in the Water Source Area of the Xiong’an New Area in North China

Investigating the attributions of evapotranspiration (ET) and gross primary production (GPP) changes is of great importance for regional, sustainable water resources and ecological management in semi-arid regions. Based on the simulation conducted during 2000–2019 by improving water-carbon coupling Distributed Time Variant Gain Model, the trends of ET and GPP were estimated and the driving factors were identified via 10 experimental scenarios in the water source area of the Xiong’an New Area in North China. The results show significant increases both in ET and GPP by 2.4 mm/a and 6.0 gC/m2/a in the region, respectively. At the annual scale, increasing precipitation dominates the ET uptrend. Air temperature, humidity and the interactive effects also contribute to the ET uptrend, and the contributions are 12.8%, 2.0% and 2.3%, respectively, while elevated atmospheric CO2 concentration (eCO2) and solar dimming lead to ET changes of about −7.2% and −12.4%, respectively. For the GPP changes, the increase in GPP is mainly caused by eCO2, increasing precipitation and rising temperature with the contributions of 56.7%, 34.8% and 27.8%, respectively. Solar dimming, humidity and windspeed contribute −6.8%, −4.8% and −3.5% of the GPP changes. Compared to climate change, land use and cover change has smaller effects on both ET and GPP for the few changes in land coverage. At the seasonal scale, ET and GPP increase to a greater extent during the growing season in spring and summer than in autumn and winter. Precipitation, temperature and eCO2 are generally the main causes for ET and GPP changes. Meanwhile, the decreasing humidity and rising temperature are dominant factors for ET and GPP increases, respectively, in winter. Furthermore, solar dimming has strong effects on ET reduction in autumn. The contribution of the interactive effects is much higher on a seasonal scale than annual scale, contributing to considerable decreases in ET and GPP in spring, increases in ET in autumn and winter, and an increase in GPP in winter. This study highlights the importance of considering water-carbon coupling on the attributions of ET and GPP changes and the differentiation of the effects by the abovementioned influential factors at annual and seasonal scales.

What roles do architectural design and on-site water servicing technologies play in the water performance of residential infill?

More than half of new urban residential developments are planned as infill in Australia's major cities. This provides an unprecedented opportunity to use innovative design and technology to address urban water challenges such as flooding, reduced water security and related infrastructure and urban heat island issues. However, infill can have positive or negative water impacts, depending on architectural design and on-site water servicing technologies implemented. In this study we asked, “What influence does residential infill development have on the local urban water cycle?” and “What roles do architectural design and technologies play?” To answer these questions, a set of 196 design-technology configurations were developed by combining 28 architectural designs and 7 on-site water-servicing technology options. The configurations represent three cases: (i) existing (EX) or before infill, (ii) business-as-usual development (BAU), and (iii) alternative development (ALT). Using the Site-scale Urban Water Mass Balance Assessment (SUWMBA) model and a set of water performance indicators, the impact of configurations on the urban water cycle was quantified. The results showed BAU, on average, increases population density, stormwater discharge, and imported water by 98%, 44% and 85%, and decreases evapotranspiration and infiltration by 53% and 34%, compared to the EX conditions. More population density (141%) with lower impacts on the urban water cycle (21% and 64% increase for stormwater discharge and imported water, and 29% and 17% reduction in evapotranspiration and infiltration) can be achieved by appropriate integration of ALT designs and technologies. Architectural design has a greater influence on urban water flows than the implementation of on-site water servicing technologies. The results have a great implication for sustainable urban water management for managing the risks associated with pluvial flooding, water insecurity, and urban heat. It also highlights the underutilised role of architects and urban planners to address urban water issues.

Urbanization Aggravates Effects of Global Warming on Local Atmospheric Drying

AbstractUrbanization is known to cause ‘Urban Heat Island’ (UHI) and elevate storm runoff. However, how urbanization influences local atmospheric moisture under global warming is not well‐understood. By examining 140 paired urban‐rural weather station data (1980–2018), this study finds significant declines in atmospheric humidity or the ‘Urban Dry Island’ (UDI) in multiple large city clusters across a large climatic gradient in China. Global warming, UHI, and reduction in local evapotranspiration and water vapor supplies all contribute to the observed UDI. The magnitude and frequency of UDI are more pronounced in humid regions than arid regions due to differences in background climate and vegetation characteristics that affect both energy and water balances at land surfaces. Mitigating the negative effects of UDI and UHI should focus on restoring the evapotranspiration power of urban ecosystems. The present empirical analyses provide new evidence and mechanistic understanding of environmental change in urban ecosystems.

Evaluation of evapotranspiration estimation under cloud impacts over China using ground observations and multiple satellite optical and microwave measurements

Evapotranspiration (ET) is an important component of the hydrological cycle and energy balance in a land-atmosphere system. Satellite remote sensing has been widely used to estimate regional and global ET, but most previous methods depend on optical measurements that are limited to cloud-free conditions. This makes ET estimation challenging under cloudy sky. Currently, evaluations of satellite ET estimation under various cloud conditions remain lacking at the regional scale. Owing to the ability to penetrate clouds, satellite passive microwave measurements are powerful tools for retrieving ET under clouds. This study evaluated a satellite microwave-based daily ET method under all sky conditions over the part of China between 18°N and 50°N from 2003 to 2010, using microwave emissivity difference vegetation index (EDVI) as the proxy of vegetation water content (VWC). Validations using the surface water balance method found that the estimated ET (EDVI-ET) had an overall small bias (6.18%) in eight river basins. EDVI-ET displayed consistent spatiotemporal patterns with global MOD16 ET, with high spatial correlation (R>0.71) and monthly temporal correlation (R>0.82) throughout four seasons. Their differences were also small (<0.56mmday−1) in forests, savannas, grass/shrubs, and croplands. Furthermore, cloud impacts on the regional ET were found to be significant and spatiotemporally heterogeneous. Both EDVI-ET and in-situ observations at seven flux towers indicated that cloud-induced reduction in daily ET could exceed 30% when the cloud cover increased by 60% (R2 of 0.42, fitting line slope of 0.80, p<0.001). Under increased cloud conditions in summer, the changes in net radiation dominated the ET over dense vegetation in southern China, while the roles of air temperature and humidity increased over water-stressed barrens and short vegetation in northwest China. VWC affected EDVI-ET under clouds in temperate transitional zones from flatlands to highlands. This study highlighted the importance of cloud impacts in satellite ET estimation.

Modeling Basin-Scale Impacts of Cultivation Practices on Cotton Yield and Water Conservation under Various Hydroclimatic Regimes

The SWAT model equipped with an improved auto-irrigation function was used to assess the impacts of cultivation practices on irrigated and dryland cotton yield and water conservation in the Texas Panhandle. Results showed the largest irrigation depth led to reductions in irrigation and crop evapotranspiration (ETc) with slightly increased cotton yields compared to the baseline scenarios under different hydroclimatic regimes. However, soil water content and surface runoff values were increased when using the largest irrigation depth. The opposite results were observed for the small irrigation depth. Early planting of cotton resulted in decreased irrigation and ETc, and increased cotton yields under both irrigated and dryland conditions, particularly in normal and wet years. By contrast, the late planting scenarios indicated the opposite for those variables. Simulated hydrologic variables were relatively stable using various maturity cultivars. Nevertheless, greater than 10% reductions in irrigated cotton yield under diverse hydroclimatic years and dryland yields during normal and wet years were identified in the long-season cotton. The opposite was determined for the short-season cotton. These outcomes suggest that a larger irrigation depth, earlier planting date, and short-season cultivar are promising cultivation practices for improving cotton yield and water conservation in the Texas Panhandle.

Understanding the Non‐Linear Response of Summer Evapotranspiration to Clouds in a Temperate Forest Under the Impact of Vegetation Water Content

AbstractUnderstanding the cloud impact on forest evapotranspiration (ET) is crucial for studying the interaction of vegetation‐cloud‐atmosphere. Combining long‐term (2003–2010) satellite passive microwave observations and in‐situ measurements, a non‐linear response of canopy‐scale ET to cloud increase was found at a temperate forest in Northeast China. As cloud increased, an initial enhancement (4%–10%) in ET occurred under less cloudy sky, while a significant reduction (&gt;20%) in ET occurred under more cloudy sky. The phenomenon existed under both high and low vegetation water content (VWC) indicated by satellite microwave emissivity difference vegetation index (EDVI). Analysis showed that this was the combined effect from the enhancement (5%–30%) in evaporative fraction (EF) and the reduction (5%–50%) in net radiation under cloud increase. Decoupling analysis based on coefficients (ρ) of path analysis model showed that enhanced EF (ρ &gt; 0.61) rather than radiation (ρ &lt; 0.47) dominated the ET enhancement under less cloudy sky, while the control of reduced radiation became stronger (ρ &gt; 0.63) and could not be compensated by increased EF (ρ &lt; 0.48) under more cloudy sky. EF enhancement under clouds was strongly correlated with the decline in canopy resistance (Rs) which was dominated by vapor pressure deficit (VPD). Higher VWC increased ET via reducing Rs and enlarging EF. This positive effect of VWC was more noticeable under less cloudy sky. Associated mechanisms could be related to the dynamic controls of plant physiology and environmental conditions induced by VWC and clouds. This study highlighted the dynamic effect of clouds and VWC on forest ET and improved our knowledge of vegetation‐cloud interactions.

Crop water and irrigation requirements of some selected crops in southwestern hydrological zone of Nigeria

This study was carried out to determine the crop water and irrigation requirement of some selected crops in southwestern Nigeria. The crops are cucumber, water melon, maize, groundnut, eggplant and tomato. Irrigation requirement and crop coefficient for each crops were determined from the interrelationships of the evapotranspiration, soil type, bulk density, field capacity and the effective root zone of the crops at the selected locations using CROPWAT for windows version 8. Soil parameters used for analysis were determined from laboratory experiment. The crop evapotranspiration and water requirement for cucumber varied from 2.52 to 7.21mm/day and 17 to 73.2mm/dec, respectively, for maize from 1.36 to 6.35mm/day and 5.1 to 63.5mm/dec respectively, for watermelon varied from 2.59 to 6.67mm/day and 25.9 to 73.3mm/dec respectively, for eggplant varied from 1.92 to 6.35 mm/day and 15.9 to 64.4mm/dec respectively. The irrigation requirement for water melon and cucumber recorded the highest value of 461.6 and 497.4mm/dec respectively, an indication that the two crops require more water for physiological activities. The reduction in the values of crop coefficient was observed during the study which could be attributed to the reduction in evapotranspiration at the late stage of growth. The findings also showed that known quantities of irrigation water could be used in producing crops optimally.