Growing Season Evapotranspiration Research Articles

Multi-year evapotranspiration estimates from four common vegetation types in a montane region of the intermountain west

Evapotranspiration (ET) is a major component of water balance and is crucial for understanding the hydrological processes, however due to highly heterogeneous distributions of soils and plants, accurate quantification of ET remains a major challenge in montane ecosystems. In this study, water use for four common vegetation communities was numerically modeled using Hydrus-1D over four continuous growing seasons (2009 to 2012), supported by a network of soil water content sensor measurements. Plant species included i) aspen (Populus tremuloides) with understory dominated by grass/forb (rudbeckia occidentalis, Bromus carinatus and Elymus trachycaulu), ii) conifer (Picea engelmannii and Abies lasiocarpa), iii) grass (dominated by B. carinatus and E. trachycaulu, et al.), and iv) sage (Artemisia tridentata). The numerical model simulated water transport within the soil and water loss from soil evaporation and plant transpiration processes. Simulations were compared with temporal and spatial dynamics of soil water content, measured at 0.1 m, 0.25 m and 0.5 m within each of 36 subplots at 30-minute intervals. Numerically simulated ET in the sage/grass meadow was compared with ET determined from a centrally-located eddy covariance tower. The simulations effectively predicted soil moisture and ET of the montane plant communities during summer dry down. Results indicate vegetation type showed no significant difference (<5 %) in ET comparing four growing seasons, except where abnormally wet conditions occurred. Four-year mean cumulative growing season ET estimates for aspen, deep-rooted conifer, shallow rooted conifer, sage, and grass were 43.0, 40.2, 28.7, 28.8 and 26.1 cm, respectively.

Divergent environmental responses of long-term variations in evapotranspiration over four grassland ecosystems in China based on eddy-covariance measurements

Understanding the long-term variation in evapotranspiration (ET) for the spatially distributed grasslands is crucial for the accurate prediction of ET response to climate change. In this study, we analyzed the interannual variability (IAV) of ET and its responses to environmental conditions at four grassland ecosystems across a wide range of climatic and biome conditions based on the long-term (9–11 years) eddy-covariance measurements. The four ecosystems encompassed the most prevalent grassland vegetations in China, containing a typical temperate steppe, an alpine meadow-steppe, an alpine shrubland meadow, and an alpine marsh meadow. The IAVs of annual ET at the typical temperate steppe and the alpine meadow-steppe were primarily affected by either change in precipitation (P) or relative humidity (RH). Leaf area index (LAI) was the dominant factor controlling the IAVs of annual and growing-season ET at the alpine shrubland meadow, and the IAV of LAI was significantly correlated with P variation. As to the alpine marsh meadow, net radiation turned to be the dominant factor for the IAV of annual ET, additionally with significant effects from water supply condition (P and RH) on the IAV of growing-season ET. Similar environmental responses were also found for the IAVs of mean surface conductance (gs) across the sites. Specifically, annual and growing-season mean gs significantly increased with increases in LAI at the alpine shrubland meadow, and appeared to be more sensitive to changes in water availability and VPD at the typical temperate steppe and the alpine meadow-steppe. Significant linear relationships were also observed among the IAVs of mean Priestley-Taylor coefficient (α = ET/ETeq, where ETeq is the equilibrium evaporation), decoupling coefficient (Ω), and gs on both the annual and growing-season basis in this study. Moreover, the variabilities of annual mean gs, Ω, and α further demonstrated the energy-limited conditions at the alpine marsh meadow, and the overall water-limited conditions at the other three grasslands. This study reveals the divergent environmental responses of long-term ET variations over grassland ecosystems, and contributes to the comprehensive understanding on the ET process and modeling efforts as well.

Effects of deforestation and forest degradation on ecosystem service indicators across the Southwestern Amazon

The Southwestern Amazon (SWA) is home to one of the Earth́s remaining intact ecosystems and a key provider of continental atmospheric moisture flows. However, the integrity of this region is threatened by global changes in climate and local to regional changes in land-use and land-cover. Here, unlike most Amazonia-land-change research which has focused primarily on deforestation, we evaluated the effects of both deforestation and forest degradation on three Ecosystem Service Indicators (ESIs) – evapotranspiration (ET), land surface temperature (LST), and precipitation (P) – at the local and watershed scale across the SWA between 2003 and 2020. We computed annual and monthly ESI differences over distinct forest conditions and buffers around disturbed areas (degraded or deforested). We also determined the influence of forest disturbance trends on ESI trends at the pixel and watershed level through a Partial Mann-Kendall approach. The results show that ESI differences among different forest conditions are statistically-significant and more pronounced during the dry season. In comparison with intact forest, monthly P rates were up to 25 % lower over any type of disturbance; whereas ET rates were up to 15 % and 48 % lower, and LST rates up to 1.6 °C and 4.4 °C higher, over degraded and deforested areas, respectively. ET and LST edge effects were only significant within buffers around some of the most heavily disturbed areas. At the pixel scale, negative trends in ET and positive trends in both LST and P were more frequently explained by forest disturbances as these trends themselves become more pronounced. ET and LST trends determined by disturbances were generally located near roads, rivers, and human settlements; and surprisingly, we found that degradation more often influences these trends than deforestation, which we attribute to the practice of converting deforested areas into crops whose growing season ET and LST rates are similar to those in natural vegetation. At the watershed scale, the analysis suggested that the climate implications of degradation and deforestation have not scaled up to this level yet; however, literature has shown that even the local impacts we report here can have important implications for areas outside of the SWA, which emphasizes the importance of continuing to conserve this remote region.

Assessing the variations of evapotranspiration and its environmental controls over a subalpine wetland valley in China

Dynamics in long-term evapotranspiration (ET) and its controlling variables are essential for understanding how a high-altitude wetlands ecosystem responds to climate change. The rising temperature is expected to agitate the regional hydrological cycle and water balance, particularly in the subalpine wetland valley of Jiuzhaigou, located in the transition zone between the northeast Qinghai-Tibet Plateau and the Sichuan Basin, Southwest China. Here, we used growing season multi-year (2013–2021) Bowen ratio data to assess the variability in ET and its key controlling parameters at different timescales in Jiuzhaigou valley. This study also explored the ratio of ET to precipitation (P). The wetland daily mean ET varied from 0.06 to 6.77 mm d−1, with a mean value of 2.64 mm d−1 for the nine years. Fluctuations in daily ET were primarily driven by available energy (net radiation, Rn), explaining 86 % of the variation. Seasonal patterns in ET were largely similar to environmental parameters, i.e., Rn, air temperature (Ta), and vapor pressure deficit (VPD), peaking in August with an interannual monthly mean value of 3.48 mm d−1. Interannual monthly mean ET had a strong positive linear relationship with Rn, Ta, and VPD, while there was no significant correlation with P on a growing season basis. Furthermore, monthly ET was shown to be regulated by Ta largely in high-temperature months and minimally in low-temperature months. The growing season ET varied interannually, and the ET to P ratio (i.e., ET/P) ranged between 0.52 and 1.16. Interannual variation in annual ET was controlled by Ta and P, which individually explained 73 and 61 % of the variation, respectively. The multiple regression model indicated that Ta and P together elucidated 92 % of the variation in annual ET. The increased sensitivity (e.g., regression slopes) of ET to P over 2014–2021 indicates that ET consumed most of P, which leads to decreasing runoff and streams drying up. This study clarifies the temporal dynamics in ET for wetlands and its environmental controls at multiple timescales. It is demonstrated that the proportion of ET could increase in response to increasing temperature without an associated increase in P, affecting local water balance. These results could potentially contribute to sustainable water management in high-altitude wetlands and environmental planning under future climate change.

Seasonal variation and controlling factors of evapotranspiration over dry semi-humid cropland in Guanzhong Plain, China

The Guanzhong Plain is a critical food production area in the Yellow River Basin that frequently suffers from water shortages. In this study, long-term (June 2013 to June 2018) water and energy fluxes were observed, and path analysis was conducted over an irrigated winter wheat (Triticum aestivum L.) / summer maize (Zea mays L.) rotation field to identify the controlling factors of evapotranspiration (ET). Total ET for each crop year ranged from 627 to 775 mm, with an average growing season ET of 398 mm for wheat and 310 mm for maize. There is significant seasonal variation in both ET and surface conductance (Gs). Daily ET varied from 0.0 to 6.0 mm d–1 for wheat and 0.0 to 6.7 mm d–1 for maize. The peak daily values of Gs were 29.5 mm s–1 for wheat and 19.5 mm s–1 for maize. The direct and indirect effects of environmental and biological factors—net radiation (Rn), surface conductance (Gs), saturation vapor pressure deficit (VPD), leaf area index (LAI), air temperature (Tair), and volumetric soil water content (VWC)—on ET were calculated using the path analysis method. Rn was determined to be the primary controlling factor of ET for both the summer maize and winter wheat growing seasons. Also, Gs was found to be another controlling factor that has more controlling power in the summer maize growing season than in the winter wheat season. VPD had a significant positive and direct effect on ET for both of the crop seasons, while it had a significant negative and indirect effect on ET through Gs in the summer maize season. VWC and Tair only directly affected the wheat ET. In addition, VWC had two significant paths that can indirectly affect ET through LAI and Gs. The revealed seasonal patterns and controlling factors of evapotranspiration in this agroecosystem provide a theoretical basis for optimizing water resources management of the Yellow River.

Attribution of growing season evapotranspiration variability considering snowmelt and vegetation changes in the arid alpine basins

Abstract. Previous studies have successfully applied variance decomposition frameworks based on the Budyko equations to determine the relative contribution of variability in precipitation, potential evapotranspiration (E0), and total water storage changes (ΔS) to evapotranspiration variance (σET2) on different timescales; however, the effects of snowmelt (Qm) and vegetation (M) changes have not been incorporated into this framework in snow-dependent basins. Taking the arid alpine basins in the Qilian Mountains in northwest China as the study area, we extended the Budyko framework to decompose the growing season σET2 into the temporal variance and covariance of rainfall (R), E0, ΔS,Qm, and M. The results indicate that the incorporation of Qm could improve the performance of the Budyko framework on a monthly scale; σET2 was primarily controlled by the R variance with a mean contribution of 63 %, followed by the coupled R and M (24.3 %) and then the coupled R and E0 (14.1 %). The effects of M variance or Qm variance cannot be ignored because they contribute 4.3 % and 1.8 % of σET2, respectively. By contrast, the interaction of some coupled factors adversely affected σET2, and the out-of-phase seasonality between R and Qm had the largest effect (−7.6 %). Our methodology and these findings are helpful for quantitatively assessing and understanding hydrological responses to climate and vegetation changes in snow-dependent regions on a finer timescale.

Impacts of alternate wetting and drying and delayed flood rice irrigation on growing season evapotranspiration

As rice production is water intensive, establishing an accurate field–scale water budget is paramount for sustainable use of local water resources. This study’s goal was to quantify and characterize half-hourly and seasonal evapotranspiration (ET) in two commercial, zero–grade rice fields in the U.S. Mid–South over three growing seasons. During each growing season, irrigation regimes for the studied fields differed between alternate wetting and drying (AWD) and delayed flooding (DF). The 2015 growing season enabled a direct comparison of the effects of AWD and DF on ET, while during the 2016 and 2017 seasons both fields were simultaneously under AWD and DF, respectively. The DF method is the region’s most common irrigation practice, and it prescribes a continuous flood to be maintained for the majority of the growing season after the plants have reached the 5-leaf growth stage (40–50 days after planting). In contrast, after holding this initial flooding for 3 weeks, AWD allows for field drying to promote the capture of seasonal rains to reduce irrigation water withdrawal and associated water pumping costs. In this study, ET was estimated using gap–filled eddy covariance observations and two variations of the Penman–Monteith equation. These methods determined growing season ET values between 560 mm and 636 mm. This study found that there were no significant differences in cumulative ET or yield when comparing AWD to DF practices. Furthermore, AWD elicited no change in ET during periods of drying when compared to DF. By this metric, AWD did not induce drought stress within the plants. We conclude that the main benefit of the AWD practice is to take advantage of seasonal rainfall to offset pumping costs and pressure on irrigation water requirements while maintaining yields comparable to conventional irrigation practices.

Evapotranspiration and its components over a rainfed spring maize cropland under plastic film on the Loess Plateau, China

Aim of study: To determine seasonal variations in evapotranspiration (ET) and its components; and ascertain the key factors controlling ET and its components in a rainfed spring maize field under plastic film.Area of study: Shouyang County in Shanxi Province on the eastern Loess Plateau, ChinaMaterial and methods: Eddy covariance system combined with micro-lysimeters and meteorological observing instruments were used in the field. The manual method was used to measure the green leaf area index (GLAI) during the growing season.Main results: In 2015 and 2016, the growing season ET accounted for 80% and 79% of annual ET, respectively. Soil evaporation (E) accounted for 36% and 33% of the growing season ET in 2015 and 2016, respectively. The daily crop coefficient increased with increasing GLAI until a threshold of ~3 m2 m−2 in the canopy-increasing stage, and decreased linearly with decreasing GLAI in the canopy-decreasing stage. At equivalent GLAI, daily basal crop coefficient and soil water evaporation coefficient were generally higher in the canopy-increasing and -decreasing stages, respectively. During the growing season, the most important factor controlling daily ET, T, and E was net radiation (Rn), followed by GLAI for daily ET and T, and soil water content at 10-cm depth for daily E; during the non-growing season, daily ET was mainly controlled by Rn.Research highlights: The daily crop coefficient and its components reacted differently to GLAI in the canopy-increasing and -decreasing stages.

Growing season evapotranspiration in boreal fens in the Athabasca Oil Sands Region: Variability and environmental controls

AbstractCurrent efforts to assess changes to the wetland hydrology caused by growing anthropogenic pressures in the Athabasca Oil Sands Region (AOSR) require well‐founded spatial and temporal estimates of actual evapotranspiration (ET), which is the dominant component of the water budget in this region. This study assessed growing season (May–September) and peak growing season (July) ET variability at a treed moderate‐rich fen and treed poor fen (in 2013–2018), open poor fen (in 2011–2014), and saline fen (in 2015–2018) using eddy covariance technique and a set of complementary environmental data. Seasonal fluctuations in ET were positively related to net radiation, air temperature and vapour pressure deficit and followed trends typical for the Boreal Plains (BP) and AOSR with highest rates in June–July. However, no strong effect of water table position on ET was found. Strong surface control on ET is evident from lower ET values than potential evapotranspiration (PET); the lowest ET/PET was observed at saline fen, followed by open fen, moderately treed fen, and heavily treed fen, suggesting a strong influence of vegetation on water loss. In most years PET exceeded precipitation (P), and positive relations between P/PET and ET were observed with the highest July ET rates occurring under P/PET ~1. However, during months with P/PET &gt; 1, increased P/PET was associated with decreased July ET. With respect to 30‐year mean values of air temperature and P in the area, both dry and wet, cool and warm growing seasons (GS) were observed. No clear trends between ET values and GS wetness/coldness were found, but all wet GS were characterized by peak growing seasons with high daily ET variability.

Ecosystem‐scale biogeochemical fluxes from three bioenergy crop candidates: How energy sorghum compares to maize and miscanthus

AbstractPerennial crops have been the focus of bioenergy research and development for their sustainability benefits associated with high soil carbon (C) and reduced nitrogen (N) requirements. However, perennial crops mature over several years and their sustainability benefits can be negated through land reversion. A photoperiod‐sensitive energy sorghum (Sorghum bicolor) may provide an annual crop alternative more ecologically sustainable than maize (Zea mays) that can more easily integrate into crop rotations than perennials, such as miscanthus (Miscanthus × giganteus). This study presents an ecosystem‐scale comparison of C, N, water and energy fluxes from energy sorghum, maize and miscanthus during a typical growing season in the Midwest United States. Gross primary productivity (GPP) was highest for maize during the peak growing season at 21.83 g C m−2 day−1, followed by energy sorghum (17.04 g C m−2 day−1) and miscanthus (15.57 g C m−2 day−1). Maize also had the highest peak growing season evapotranspiration at 5.39 mm day−1, with energy sorghum and miscanthus at 3.81 and 3.61 mm day−1, respectively. Energy sorghum was the most efficient water user (WUE), while maize and miscanthus were comparatively similar (3.04, 1.75 and 1.89 g C mm−1 H2O, respectively). Maize albedo was lower than energy sorghum and miscanthus (0.19, 0.26 and 0.24, respectively), but energy sorghum had a Bowen ratio closer to maize than miscanthus (0.12, 0.13 and 0.21, respectively). Nitrous oxide (N2O) flux was higher from maize and energy sorghum (8.86 and 12.04 kg N ha−1, respectively) compared with miscanthus (0.51 kg N ha−1), indicative of their different agronomic management. These results are an important first look at how energy sorghum compares to maize and miscanthus grown in the Midwest United States. This quantitative assessment is a critical component for calibrating biogeochemical and ecological models used to forecast bioenergy crop growth, productivity and sustainability.

How Film Mulch Increases the Corn Yield by Improving the Soil Moisture and Temperature in the Early Growing Period in a Cool, Semi-Arid Area

Film mulch increases the crop grain yield via topsoil moisture and temperature improvement in cool, semi-arid areas, but little is known about the role of the hydrological and thermic relationship between early and later crop growth seasons in the improving grain yield. We conducted a field experiment to compare polyethylene film mulching (PM) with no mulching (CK) in 2014 and 2015 on the semi-arid Loess Plateau of China. Compared to CK, PM decreased evapotranspiration before the twelve-leaf stage (V12), but increased evapotranspiration after the V12 stage, and significantly increased the topsoil temperature before the six-leaf stage (V6) and the accumulation of soil growing degree days. Corn plants with PM treatment reached the V6 stage earlier, significantly enhancing the contemporary dry matter accumulation. The harvest index, 100-grain weight, and grain yield significantly increased in PM relative to CK in both years. The growing period to the whole growing season evapotranspiration ratio had a negative correlation with the grain yield before the V12 stage, but a positive correlation after the V12 stage. The grain yield had a negative correlation with the air growing degree days (GDDair) before the V6 stage, but positive correlation from silking to harvest. Conclusively, film mulch promoted the early development of maize via an increased soil temperature before the V6 stage, saved soil water before the V12 stage, resulted in a longer grain-filling period, and increased the GDDair and evapotranspiration during the grain-filling period, which is key to increasing the maize yield.

Spatio‐temporal calibration of Hargreaves‐Samani model to estimate reference evapotranspiration across U.S. High Plains

AbstractTemperature‐based grass‐reference evapotranspiration (ETo) estimation methods (e.g., Hargreaves−Samani [HS] model) present advantages over combination‐based methods that require full‐suite weather data. The U.S. High Plains region has scarce and short‐term full‐suite weather sites. This data scarcity presents challenges for combination‐based ETo estimation. The performance of HS model against the American Society of Civil Engineers (ASCE) standardized Penman−Monteith (PM) model was assessed using long‐term data at 124 full‐suite weather sites across nine states in the U.S. High Plains. The HS model underestimated ETo at arid (mean bias error [MBE] = −1.68 mm d−1), semi‐arid (MBE = −0.34 mm d−1), and dry subhumid sites (MBE = −0.16 mm d−1) and overestimated ETo at humid sites (MBE = 0.14 mm d−1). There was a significant relationship (p &lt; .01) between HS model performance and aridity index. The HS model performed better (27% lower root mean squared difference [RMSD]) in summer months than the rest of the year at semi‐arid and dry subhumid sites. The model performance was non‐ideal during the summer months in subhumid climates. Spatio‐temporal annual zonal (climate division), monthly zonal, annual site‐specific, and monthly site‐specific calibration resulted in 12, 16, 20, and 26% reduction in RMSD and 11, 16, 17, and 23% reduction in relative error, respectively. Monthly site‐specific calibration performed the best and was used to quantify annual and growing season ETo across the region. The research characterized performance patterns of the HS model over an important agroecosystem‐dominated region. Practical data‐driven strategies were proposed to better estimate PM ETo using limited weather data at any given site (with similar aridity) and time of the year.

Estimating Growing Season Evapotranspiration and Transpiration of Major Crops over a Large Irrigation District from HJ-1A/1B Data Using a Remote Sensing-Based Dual Source Evapotranspiration Model

Crop evapotranspiration (ET) is the largest water consumer of agriculture water in an irrigation district. Remote sensing (RS) technique has provided an effective way to map regional ET using various RS-based ET models over the past several decades. To map growing season ET of different crops and partition ET into evaporation (E) and transpiration (T) at regional scale, appropriate ET models should be further integrated with crop distribution maps in different years and crop growing seasons determined for each crop pixel. In this study, a hybrid dual-source scheme and trapezoid framework-based ET Model (HTEM) fed with HJ-1A/1B data was applied in Hetao Irrigation District (HID) of China from 2009 to 2015 to map crop growing season ET and T at 30 m resolution. The HTEM model with HJ-1A/1B data performed well in estimating ET in HID, and the finer spatial resolution of model input data can improve the estimation accuracy of ET. Combined with the annual crop planting map identified in previous study, and crop growing seasons determined from fitted Normalized Difference Vegetation Index (NDVI) curves for crop pixels, the spatial and temporal variations of growing season ET and T of major crops (maize and sunflower) were examined. The results indicate that ET and T of maize and sunflower reach their minimum values in the southwest HID with smaller crop planting density, and reach their maximum values in northwest HID with higher crop planting density. Over the study period with a decreasing trend of available irrigation water, ET and T in maize and sunflower growing seasons show decreasing trends, while ratios of T/ET show increasing trends, which implies that the adverse effect of decreased irrigation water diversion on crop growth is diminished due to the favorable portioning of E and T in cropland of HID. In addition, the calculation results of crop coefficients show that there is water stress to crop growth in the study area. The present results are helpful to better understand the spatial pattern of crop water consumption and water stress of different crops during crop growing season, and provide the basis for optimizing the spatial distribution of crop planting with less water consumption and more crop yield.

Transpiration from subarctic deciduous woodlands: Environmental controls and contribution to ecosystem evapotranspiration

AbstractPotential land–climate feedbacks in subarctic regions, where rapid warming is driving forest expansion into the tundra, may be mediated by differences in transpiration of different plant functional types. Here, we assess the environmental controls of overstorey transpiration and its relevance for ecosystem evapotranspiration in subarctic deciduous woodlands. We measured overstorey transpiration of mountain birch canopies and ecosystem evapotranspiration in two locations in northern Fennoscandia, having dense (Abisko) and sparse (Kevo) overstories. For Kevo, we also upscale chamber‐measured understorey evapotranspiration from shrubs and lichen using a detailed land cover map. Subdaily evaporative fluxes were not affected by soil moisture and showed similar controls by vapour pressure deficit and radiation across sites. At the daily timescale, increases in evaporative demand led to proportionally higher contributions of overstorey transpiration to ecosystem evapotranspiration. For the entire growing season, the overstorey transpired 33% of ecosystem evapotranspiration in Abisko and only 16% in Kevo. At this latter site, the understorey had a higher leaf area index and contributed more to ecosystem evapotranspiration compared with the overstorey birch canopy. In Abisko, growing season evapotranspiration was 27% higher than precipitation, consistent with a gradual soil moisture depletion over the summer. Our results show that overstorey canopy transpiration in subarctic deciduous woodlands is not the dominant evaporative flux. However, given the observed environmental sensitivity of evapotranspiration components, the role of deciduous trees in driving ecosystem evapotranspiration may increase with the predicted increases in tree cover and evaporative demand across subarctic regions.

辽河三角洲湿地生长季蒸散量时空格局及影响因素

辽河三角洲湿地生长季蒸散量时空格局及影响因素

Longer term effects of biological control on tamarisk evapotranspiration and carbon dioxide exchange

AbstractBiological control of Tamarix spp. (tamarisk) with Diorhabda spp. (tamarisk beetle) was initiated in several states in the Western United States in 2001. We analysed 12 years of evapotranspiration (ET), net ecosystem production (NEP), and beetle abundance data from a tamarisk‐invaded site in Western Nevada along the Truckee River. Diorhabda carinulata (northern tamarisk beetle) appeared at the site in 2007. Large beetle outbreaks and associated defoliation of the tamarisk occurred in 2008 and 2009, then the beetle population was highly variable from year to year. Since 2016, the beetle population declined. Growing season ET noticeably declined from direct beetle herbivory in 2008, 2009, and 2010, but the decline in ET was seasonally transient as trees regrew leaves. In 2012 and 2013, total growing season ET was low, likely due to the combined effects of drought and beetle herbivory pressure. Total seasonal ET losses and NEP were primarily driven by annual precipitation with higher values in wetter years and reduced values when precipitation fell below 100 mm. In the last 2 years of the study, 2017–2018, there were few to no beetles observed at the site, and we measured increased tamarisk leaf area index, ET, and NEP. Since 2010 at the study site, no further releases of the beetles have occurred due to wildlife concerns, and subsequent declines in beetle populations where such that the “outbreak” conditions apparently required to impair tamarisk physiological function and significantly reduce ET have not occurred. ET and photosynthesis were highly correlated (r2 ≥ .91) to the Landsat‐satellite normalized difference vegetation index (NDVI). Using a relationship between growing season ET and NDVI, we estimated ET for five additional tamarisk sites along several southwestern U.S. rivers. In the 2005 to 2018 analysis period, NDVI‐estimated ET declined at all sites after beetle arrival with three sites showing a recovery in pre‐beetle ET rates in subsequent years. At the other three sites, ET rates have not recovered to pre‐beetle levels.

Long‐term evapotranspiration rates for rainfed corn versus perennial bioenergy crops in a mesic landscape

AbstractPerennial cellulosic crops are promoted for their potential contributions to a sustainable energy future. However, a large‐scale perennial bioenergy production requires extensive land use changes through diversion of croplands or conversion of uncultivated lands, with potential implications for local and regional hydrology. To assess the impact of such land use conversions on ecosystem water use, we converted three 22 year‐old Conservation Reserve Program (CRP) grasslands and three 50+ year‐old conventionally tilled corn‐soybean crop fields (AGR) to either no‐till continuous maize (corn) or perennial (switchgrass or restored prairie) bioenergy crops. We also maintained one CRP grassland without conversion. We measured evapotranspiration (ET) rates on all fields for 9 years using eddy covariance methods. Results show that: (a) mean growing‐season ET rates for perennial crops were similar to the ET rate of the corn they replaced at the previously cultivated (AGR) field but ET rates for perennial crops at CRP fields were 5–9% higher than ET rate for corn on former CRP fields; and (b) mean nongrowing season ET rates for perennial fields were 11–15% lower than those for corn fields, regardless of land use history. On an annual basis, mean ET rates for perennial crops tended to be lower (4–7%) than ET rate of the corn that they replaced at AGR fields but ET rates for perennial crops and corn at CRP fields were similar. Over 9 years, mean ET rates for the same crop across land use histories were remarkably similar for corn, whereas for the perennial crops they were 4–10% higher at former CRP than at former AGR fields, mainly due to differences in growing season ET. Over the 9 years and across all fields, ET returned ~60% of the precipitation back to the atmosphere. These findings suggest that large‐scale substitution of perennial bioenergy crops for rainfed corn in mesic landscapes would have little if any (0 to −3%) impact on terrestrial water balances.

Controls on ecosystem water-use and water-use efficiency: Insights from a comparison between grassland and riparian forest in the northern Great Plains

Within the grassland-dominated landscape of the North American Great Plains, riparian forest ecosystems exist along river floodplains. We compared cumulative evapotranspiration (ET) and ecosystem water-use efficiency (WUE) between a cottonwood forest and a nearby native grassland ecosystem in southern Alberta, using eddy covariance measurements during May-September (growing season) of three study years. Our objective was to test predictions about mechanistic controls on ecosystem water-use, and to provide insights into the amount of alluvial groundwater and stored soil water required to support a healthy riparian forest within the Great Plains biome. Grassland ET was dependent on precipitation inputs during the growing season. Cumulative growing season ET at the cottonwood site (375–451 mm) exceeded grassland ET (111–213 mm) by 2.1- to 3.4-fold depending on study year, despite slightly higher WUE in the cottonwood ecosystem. The difference in cumulative ET between ecosystems ranged from 238 to 264 mm in different years, in a region that normally receives 258 mm of cumulative precipitation during May-September. The large ET at the cottonwood site was caused by two-fold higher LAI, and associated greater canopy conductance than was apparent at the grassland site. The additional soil water required for the higher cottonwood ET was supplied by access to alluvial groundwater, which is recharged by river water, and was also supported by a larger soil volume to store water from precipitation and river flooding inputs. These factors resulted in a relatively long interval for cottonwood photosynthetic gas exchange that was consistent among years despite widely different environmental conditions, while the grassland had shorter growing season lengths that were constrained further as precipitation and soil moisture declined among years. Our analyses contribute to understanding the water requirements of these contrasting ecosystems and will help to improve management procedures for regulating river flow rates in order to sustain healthy riparian cottonwood ecosystems.

The Sensitivity of Land-Atmosphere Coupling to Modern Agriculture in the Northern Midlatitudes.

Modern agricultural land cover and management are important as regional climate forcings. Previous work has shown that land cover change can significantly impact key climate variables, including turbulent fluxes, precipitation, and surface temperature. However, fewer studies have investigated how intensive crop management can impact background climate conditions, such as the strength of land-atmosphere coupling and evaporative regime. We conduct sensitivity experiments using a state-of-the-art climate model with modified vegetation characteristics to represent modern crop cover and management, using observed crop-specific leaf area indexes and calendars. We quantify changes in land-atmosphere interactions and climate over intensively cultivated regions situated at transitions between moisture- and energy-limited conditions. Results show that modern intensive agriculture has significant and geographically varying impacts on regional evaporative regimes and background climate conditions. Over the northern Great Plains, modern crop intensity increases the model simulated precipitation and soil moisture, weakening hydrologic coupling by increasing surface water availability and reducing moisture limits on evapotranspiration. In the U.S. Midwest, higher growing season evapotranspiration, coupled with winter and spring rainfall declines, reduces regional soil moisture, while crop albedo changes also reduce net surface radiation. This results overall in reduced dependency of regional surface temperature on latent heat fluxes. In central Asia, a combination of reduced net surface energy and enhanced pre-growing season precipitation amplify the energy-limited evaporative regime. These results highlight the need for improved representations of agriculture in global climate models to better account for regional climate impacts and interactions with other anthropogenic forcings.

Water balance of pine forests: Synthesis of new and published results

The forest hydrologic cycle is expected to have important feedback responses to climate change, impacting processes ranging from local water supply and primary productivity to global water and energy cycles. Here, we analyzed water budgets of pine forests worldwide. We first estimated local water balance of forests dominated by two wide-ranging species: Pinus taeda (36 °N) and Pinus sylvestris (64 °N). In these stands, growing season evapotranspiration (ET) was largely insensitive to inter-annual variation of precipitation (P), consistent with the insensitivity of canopy transpiration to P. Extending the analyses to include published data from 117 studies on 27 pine species, we found that pine forests annually use ∼66% ± 17% (SD) of P as ET, regardless of climatic regime, leaving a third of P as runoff to downstream aquatic ecosystems and users. However, during the growing season, pine forests used more water as ET than P in regions where P ≤ 326 ± 39 (SE) mm. Forests in regions of low growing season P exist in their current state only where the rooting depth is sufficient to supply trees with water from soil storage in addition to P, and these forests are likely to support only ephemeral streams that dry down during the growing season. Thus, globally, water use by pine forests is adapted to mean annual P, but shows a limited capacity to respond to inter-annual variability in P. Forests with a small buffer of growing season water availability (P + soil water storage - ET), are likely to be most sensitive to variation in P regimes, changing canopy leaf area, tree density, and species composition depending on the degree, direction and persistence of the change in P.