Growing Season Evapotranspiration Research Articles

Quantifying potential yield and lodging-related yield gaps for irrigated spring wheat in sub-tropical Australia

Approximately 15% of global wheat production (40% of the wheat produced by developing countries) occurs in irrigated, low-rainfall regions of the subtropics. Irrigated wheat is uncommon in sub-tropical Australia because cotton is more profitable, but high grain prices in 2008 triggered a record area of irrigated wheat production. Unfortunately, widespread lodging occurred and yields were lower than expected, although the losses directly attributable to lodging were unknown.Crop models have not been previously validated for calculating potential yield and yield gaps of mechanised, lodging-susceptible production systems. This study aimed to determine potential yield, farmer-realised yield, and water use requirements of lodging-prone irrigated wheat in sub-tropical Australia, using field monitoring and the APSIM simulation model. Yield data were obtained in 2008 and 2009 from 16 commercial irrigated wheat fields and multiple treatments from four irrigated experiments, and used to assess APSIM's ability to predict grain yield of irrigated wheat.APSIM predicted biomass production satisfactorily in 2008 but substantially over-predicted grain yield of lodged fields. The mean difference (yield gap) between APSIM-estimated potential yield and farmer-realised yield was 1.0tha−1 in non-lodged fields, and 2.7tha−1 in lodged fields. The average effect of lodging was therefore a decrease of 1.7tha−1; the difference between the yield gap calculated for lodged and non-lodged fields. Yield gaps in lodged fields were highly correlated with soil N content at sowing, with most of the lodged fields having more than 300kgha−1 of soil N at sowing to a depth of 90cm.In 2009, management of commercial fields aimed to avoid lodging by using the canopy management technique of in-season N application. APSIM generally under-predicted biomass production and yield in these fields, suggesting that the N uptake parameters in APSIM may require adjustment. Observed yields from fields of a quick-maturing cultivar that experienced little lodging were simulated accurately when N was assumed to be non-limiting.The simulation study found that the potential yield of irrigated spring wheat in sub-tropical Australia is currently between 8 and 9tha−1, and average growing season evapotranspiration of such crops was approximately 500–550mm. Potential yield of furrow-irrigated fields with one or two-metre wide beds were 16% and 8% lower, due to the area occupied by the 60cm wide unsown furrows between irrigation beds.This study demonstrated the value of using validated simulation models for estimating potential yield and yield gap analysis. Yield gap analysis also enabled the identification of agronomic characteristics (high levels of soil residual N) which probably contributed to lodging. Further investigation is required to determine whether alternative varieties or the canopy management technique of in-season N application can increase the potential yield of irrigated spring wheat above the levels reported here.

Hourly and Daily Single and Basal Evapotranspiration Crop Coefficients as a Function of Growing Degree Days, Days After Emergence, Leaf Area Index, Fractional Green Canopy Cover, and Plant Phenology for Soybean

<abstract><title><italic>Abstract.</italic></title> Hourly evapotranspiration (ET) crop coefficients (K_c) are needed to optimize the effectiveness and efficiency of high-frequency micro- and sprinkler irrigation practices involving the application of water multiple times a day. However, not much is known about the daily and seasonal patterns and magnitudes in hourly K_c values for soybean. In addition, locally developed K_c values are necessary for more robust within-season irrigation management, crop ET estimation, and water balance analyses. Hourly and daily K_c functions were developed for soybean in south-central Nebraska through extensive field research. Actual crop evapotranspiration (ET_a) was measured using a Bowen ratio energy balance system. Daily crop coefficients were calculated as K_c = ET_a/ET_ref, wherein reference (potential) evapotranspiration (ET_ref) was calculated using the Penman-Monteith equation with a fixed canopy resistance for both alfalfa-reference (ET_r) and grass-reference (ET_o) surfaces. The K_c values were derived in two forms: (1) a single (normal or average K_c) K_cr based on ET_r, and K_co based on ET_o; and (2) a basal coefficient (K_cbr) based on ET_r, and K_cbo based on ET_o. The seasonal patterns of variation of K_cr, K_co, K_cbr, and K_cbo were examined on five different temporal base scales: days after emergence (DAE), cumulative growing degree days (GDD), leaf area index (LAI), fractional green canopy groundcover (CC), and plant phenology (V and R stages). The 2007 and 2008 growing season ET_a totals were 535 and 514 mm, respectively. Extreme hourly K_c values were frequently observed in the early morning and late afternoon hours when ET_a was very low relative to ET_r and ET_o. Daily means of the 10 to 13 hourly values computed for K_cr ranged from 0.25 to 1.06 in 2007 and from 0.15 to 1.02 in 2008, whereas those computed for K_co ranged from 0.39 to 1.37 in 2007 and from 0.22 to 1.29 in 2008. Daily K_cr and K_co values calculated based on daily data ranged from 0.20 to 1.12 and from 0.27 to 1.47, respectively. Comparison of all daily means of hourly coefficients with the corresponding daily coefficients in one-to-one graphs and zero-origin based regression of the former on the latter revealed linear regression coefficients of 0.92 (2007 K_cr), 0.95 (2008 K_cr), 0.96 (2007 K_co), and 0.97 (2008 K_co), with R² values of 0.78 or better. On average, hourly K_c values were about 4% to 8% lower that the corresponding daily values. Substantial diurnal variability was observed in K_co and K_cr measured during daylight hours (ranging from 0.1-0.2 to 1.5-1.6) from early morning to late afternoon (8:00 to 18:00), and the range of variability was substantially dependent on the coincident V and R stages. The relationship between K_c and LAI was best represented by two regression trend lines: one representing crop development from its beginning up to the start of senescence, and the other representing crop development thereafter. A similar break in the regression trend line was observed in the relationship between basal K_c and GDD. In contrast, the relationship between K_c and fractional CC was not biphasic and could be modeled with one regression trend line. The FAO-56 tabulated K_co values and those measured in this research were significantly different (p < 0.05). Thus, the FAO-56 values, if used for south-central Nebraska soil, climate, and management practices or similar conditions, would not be able to provide accurate ET_a and crop water requirement estimates. Because this research proved that K_co and K_cr values are not constant during the day from dawn to dusk, using daily average K_co or K_cr values would not be able to provide robust and precise determination of crop irrigation requirements for irrigation practices delivered more than once per day. The crop coefficients developed in this research as a function of several base scales should provide crop consultants, extension service personnel, agronomists, irrigation practitioners, and other irrigation and water management professionals with robust and accurate methods for choosing and applying crop coefficients to be used for more precise determination of ET_a and water requirements, thus leading to more efficient and effective seasonal soybean irrigation management.

Effect of long-term water table manipulation on peatland evapotranspiration

Continuous measurements of ecosystem scale evapotranspiration (ET) were obtained using the eddy covariance method over the 2010 and 2011 growing seasons (May–September) at three adjacent peatlands that have undergone long-term water table manipulation. The three (wet, dry and intermediate) sites represent peatlands along a hydrological gradient, with different average depths to water table (WTD) and different resulting vegetation and microform assemblages. The 2010 growing season was warmer and wetter than normal, while 2011 conditions were near normal. The difference in maximum daily ET values (95th percentiles) between sites were greater in 2010 (3.14mmd−1–4.17mmd−1) compared to 2011 (3.68mmd−1–3.95mmd−1), yielding cumulative growing season ET that followed the wet to dry gradient in both 2010 and 2011. Synoptic weather conditions (i.e. air temperature, vapour pressure deficit, and incoming solar radiation, etc.) could not explain differences in ET between sites due to their proximity to one another. Peat surface wetness was more spatially homogeneous at the wet site due to small average microtopographic variations (0.15m) compared to the intermediate (0.30m) and dry (0.41m) sites. Although average Bowen ratios were less than one at all three sites, greater surface wetness and heating at the wettest site lead to differences in energy partitioning, with higher average Bowen ratios at the sites with a shallow average WTD. No significant relation between normalized ET and WTD was found at any of the sites that were consistent across both study years. In addition, the lack of a relation between ET and near-surface moisture suggests that the unsaturated hydraulic conductivity and the boundary layer resistance created by the vascular canopy combined with low surface roughness limits evaporative losses from the peat surface. This study suggests that the low ET of a dry site compared to a wet site may be due to the impact of a long-term change in WTD on leaf area and the relative distribution of plant functional groups.

Soil sorptivity enhancement with crop residue accumulation in semiarid dryland no-till agroecosystems

Water capture and precipitation use efficiency are of great importance in dryland cropping systems because the world's dependence on food produced in dryland areas continues to increase. Growing season evapotranspiration potential greatly exceeds growing season precipitation rates in dryland areas, creating a water deficit for crops. Management practices that positively impact soil physical properties increase the potential for soils to capture water. One way to assess the ability of soils to capture water is through the measurement of sorptivity. Sorptivity is defined as the cumulative infiltration proportionality constant and is governed by surface soil physical properties such as texture, degree of aggregation and aggregate stability. A study was conducted to determine how crop residue accumulation after 12years of no-till management affects surface soil sorptivity under semi-arid dryland conditions and how sorptivity is related to surface soil physical properties known to be related to crop residue accumulation. Surface soil sorptivity, bulk density, porosity (total and effective) and aggregation measurements were made across cropping systems and soil positions representing a wide gradient of crop residue accumulation at 3 sites in eastern Colorado. Results show that increasing crop residue accumulation will have the indirect effect of increased sorptivity via improvements in soil aggregation, bulk density, and porosity that are conducive to water infiltration. Management practices that result in greater amounts of crop residue returned to the soil system lead to beneficial soil physical properties that increase water sorptivity, greatly reducing the potential for runoff and erosion, and thereby increase the precipitation use efficiency of the system.

Calibrating a soil–vegetation–atmosphere transfer model with remote sensing estimates of surface temperature and soil surface moisture in a semi arid environment

Summary A series of numerical experiments has been designed to investigate how effective satellite estimates of radiometric surface temperatures and soil surface moisture are for calibrating a Soil–Vegetation–Atmosphere Transfer (SVAT) model. Multi–objective calibration based on error minimization of temperature and soil moisture model outputs is performed in a semi–arid environment. Model accuracy when calibrated using in situ versus satellite objectives is explored in detail. Observational meteorological datasets from the African Monsoon Multidisciplinary Analysis (AMMA) were used to force a column model during a growing season in Mali. Fourier Amplitude Sensitivity Test (FAST) revealed the most sensitive parameters to model outputs. Parameters found sensitive were subsequently optimized in a series of model calibrations to reveal trade-offs between model objectives. Our main findings are (1) the SVAT model performs well in the semi–arid environment, but underestimates peak growing season evapotranspiration and overestimates soil moisture, (2) most of the parameters important for flux estimates can be constrained using surface temperature and soil surface moisture with the three exceptions: root depth, the extinction coefficient and unstressed stomatal resistance, (3) flux simulations are improved when the model is calibrated using in situ surface temperature and soil surface moisture versus satellite estimates.

Linear models for estimating annual and growing season reference evapotranspiration using averages of weather variables

AbstractWe develop linear regression equations to estimate location‐specific average annual reference evapotranspiration (ETo) using one or more of annual averages of: incoming solar radiation (Rs), air temperature (T), relative humidity (RH), and wind speed (U). We also provide two sets of equations to estimate growing season ETo, either using one or more of annual averages of Rs, T, RH, and U, or using growing season averages of the same variables. The equations are developed using the FAO‐56 Penman–Monteith model ETo estimates as a reference. Supporting weather data to develop the regression equations were from 102 locations (494 station‐years) across the contiguous United States. The models were tested with additional data from 32 stations (114 station‐years). To illustrate potential applications of the regression models, we estimate spatial patterns of annual ETo and growing season ETo across the contiguous United States using existing spatial datasets of annual averages of Rs, T, RH, and U. Other applications of the models provided may include rapid assessments of historical annual and growing season ETo, evaluation of past ETo trends, or evaluation of ETo projected trends based on output from global climate models. Copyright © 2012 Royal Meteorological Society

Improved methods for estimating monthly and growing season ET using METRIC applied to moderate resolution satellite imagery

AbstractSatellite‐based algorithms based on surface energy balance are now routinely applied to produce evapotranspiration (ET) products on an operational basis for use in water resources management. Landsat satellite imagery, or imagery with similar spatial resolution, is commonly used to produce estimates of ET at field scale because of the presence of an onboard thermal imager and the high spatial resolution of the imagery. The downside of using high‐resolution imagery is less frequent image acquisition. As a result, monthly and ultimately seasonal ET estimates may be based on only one or two satellite image snapshots per month. A potential shortfall in basing integrated ET averages on periodic snapshots from satellites is that local or regional precipitation events antecedent to the satellite images may unduly dominate the ET image, or conversely, effects may be absent from the image, and therefore the image‐based product may not represent evaporation from rainfall averaged over the monthly period. Methods for accounting for precipitation events when interpolating from daily to seasonal or longer periods are presented, including a recently developed soil‐water balance procedure that adjusts the ET derived from the satellite overpass date for background evaporation from soil caused by rainfall over monthly or longer integration periods. The result of the adjustment is an ET image and consequently final ET map that better represents the average evaporative conditions over the period. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Comparisons of energy balance and evapotranspiration between flooded and aerobic rice fields in the Philippines

Abstract The seasonal and annual variability of sensible heat flux (H), latent heat flux (LE), evapotranspiration (ET), crop coefficient (Kc) and crop water productivity (WPET) were investigated under two different rice environments, flooded and aerobic soil conditions, using the eddy covariance (EC) technique during 2008–2009 cropping periods. Since we had only one EC system for monitoring two rice environments, we had to move the system from one location to the other every week. In total, we had to gap-fill an average of 50–60% of the missing weekly data as well as those values rejected by the quality control tests in each rice field in all four cropping seasons. Although the EC method provides a direct measurement of LE, which is the energy used for ET, we needed to correct the values of H and LE to close the energy balance using the Bowen ratio closure method before we used LE to estimate ET. On average, the energy balance closure before correction was 0.72 ± 0.06 and it increased to 0.99 ± 0.01 after correction. The G in both flooded and aerobic fields was very low. Likewise, the energy involved in miscellaneous processes such as photosynthesis, respiration and heat storage in the rice canopy was not taken into consideration. Average for four cropping seasons, flooded rice fields had 19% more LE than aerobic fields whereas aerobic rice fields had 45% more H than flooded fields. This resulted in a lower Bowen ratio in flooded fields (0.14 ± 0.03) than in aerobic fields (0.24 ± 0.01). For our study sites, evapotranspiration was primarily controlled by net radiation. The aerobic rice fields had lower growing season ET rates (3.81 ± 0.21 mm d−1) than the flooded rice fields (4.29 ± 0.23 mm d−1), most probably due to the absence of ponded water and lower leaf area index of aerobic rice. Likewise, the crop coefficient, Kc, of aerobic rice was significantly lower than that of flooded rice. For aerobic rice, Kc values were 0.95 ± 0.01 for the vegetative stage, 1.00 ± 0.01 for the reproductive stage, 0.97 ± 0.04 for the ripening stage and 0.88 ± 0.03 for the fallow period, whereas, for flooded rice, Kc values were 1.04 ± 0.04 for the vegetative stage, 1.11 ± 0.05 for the reproductive stage, 1.04 ± 0.05 for the ripening stage and 0.93 ± 0.06 for the fallow period. The average annual ET was 1301 mm for aerobic rice and 1440 mm for flooded rice. This corresponds to about 11% lower total evapotranspiration in aerobic fields than in flooded fields. However, the crop water productivity (WPET) of aerobic rice (0.42 ± 0.03 g grain kg−1 water) was significantly lower than that of flooded rice (1.26 ± 0.26 g grain kg−1 water) because the grain yields of aerobic rice were very low since they were subjected to water stress. The results of this investigation showed significant differences in energy balance and evapotranspiration between flooded and aerobic rice ecosystems. Aerobic rice is one of the promising water-saving technologies being developed to lower the water requirements of the rice crop to address the issues of water scarcity. This information should be taken into consideration in evaluating alternative water-saving technologies for environmentally sustainable rice production systems.

Quantifying structural and physiological controls on variation in canopy transpiration among planted pine and hardwood species in the southern Appalachians

AbstractRecent studies have shown that planted pine stands exhibit higher evapotranspiration (ET) and are more sensitive to climatic conditions compared with hardwood stands. Whether this is due to management and stand effects, biological effects or their interaction is poorly understood. We estimated growing season canopy‐ and sap flux‐scaled leaf‐level transpiration (Ec and EL) in five major overstory species over 3 years. Four hardwood species, Liriodendron tulipifera, Carya spp., Quercus rubra and Quercus prinus, were measured in an unmanaged watershed. Pinus strobus was measured in an adjacent planted pine watershed. We hypothesized that (1) species would differ in EL and stomatal conductance (GS), and their relationship with vapour pressure deficit (D); and (2) differences in growing season ET between the stands would result primarily from the differences in interception (Ei). Growing season ET in the planted pine stand exceeded hardwood ET by twofold during all 3 years. Transpiration and Ei contributed similarly to the ET difference, suggesting that physiological differences were equally as important as structural factors to the overall difference in ET. Among species, mean EL and GS differed significantly, as did their relationship with D. EL and GS of oaks and hickories were least responsive to changing D, while L. tulipifera and P. strobus were most responsive. This species‐level understanding of variation in EL and GS and their interactions with climatic driving variables has important implications for predicting watershed‐level responses to stand management, species invasion and loss, and climate variability. Published in 2010. This article is a US Government work and is in the public domain in the USA.

A general predictive model for estimating monthly ecosystem evapotranspiration

AbstractAccurately quantifying evapotranspiration (ET) is essential for modelling regional‐scale ecosystem water balances. This study assembled an ET data set estimated from eddy flux and sapflow measurements for 13 ecosystems across a large climatic and management gradient from the United States, China, and Australia. Our objectives were to determine the relationships among monthly measured actual ET (ET), calculated FAO‐56 grass reference ET (ETo), measured precipitation (P), and leaf area index (LAI)—one associated key parameter of ecosystem structure. Results showed that the growing season ET from wet forests was generally higher than ETo while those from grasslands or woodlands in the arid and semi‐arid regions were lower than ETo. Second, growing season ET was found to be converged to within ± 10% of P for most of the ecosystems examined. Therefore, our study suggested that soil water storage in the nongrowing season was important in influencing ET and water yield during the growing season. Lastly, monthly LAI, P, and ETo together explained about 85% of the variability of monthly ET. We concluded that the three variables LAI, P, and ETo, which were increasingly available from remote sensing products and weather station networks, could be used for estimating monthly regional ET dynamics with a reasonable accuracy. Such an empirical model has the potential to project the effects of climate and land management on water resources and carbon sequestration when integrated with ecosystem models. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Spatial and temporal controls on watershed ecohydrology in the northern Rocky Mountains

Vegetation water stress plays an important role in the movement of water through the soil‐plant‐atmosphere continuum. However, the effects of water stress on evapotranspiration (ET) and other hydrological processes at the watershed scale remain poorly understood due in part to spatially and temporally heterogeneous conditions within the watershed, especially in areas of mountainous terrain. We used a spatially distributed model to understand and evaluate the relationship between water stress and ET in a forested mountain watershed during the snow‐free growing season. Vegetation water stress increased as the growing season progressed, due to continued drying of soils, and persisted late into the growing season, even as vapor pressure deficit decreased with lower temperatures. As a result, ET became decoupled from vapor pressure deficit and became increasingly dependent on soil moisture later in the growing season, shifting from demand limitation to supply limitation. We found water stress and total growing season ET to be distributed nonuniformly across the watershed due to interactions between topography and vegetation. Areas having tall vegetation and low topographic index experienced the greatest water stress, yet they had some of the highest evapotranspiration rates in the watershed.

Evapotranspiration (ET) and regulating mechanisms in two semiarid Artemisia-dominated shrub steppes at opposite sides of the globe

Understanding the regulation of ecosystem ET is crucial to comprehend present and potential future vegetation and water availability in arid environments. We compared ET data from eddy-covariance flux measurements in two semiarid shrub-dominated ecosystems. Located in the Kubuqi Desert (KD, Inner Mongolia, China) and the Sierra Madre (SM, Wyoming, USA), the ecosystems shared similarities in latitude, climate and vegetation. We analyzed ET at KD and SM for the years 2006–2007 and 2004–2005, respectively, and tested regulatory functions of climate variables through modifications to the widely used FAO-Penman–Monteith model. Important modifications included the insertion of soil water potential (Ψs) and the removal of vapor pressure deficit (VPD) and wind speed ( u). The modifications improved daily estimates of growing season ET as they reduced deviations between model and measurement to 1–33%. According to model results, available energy ( R n − G) and Ψs were the main drivers of ET during growing seasons. VPD and u influenced ET mainly after precipitation events and at low Ψs (i.e., Ψs close to zero). Although our empirical model still requires validation through data from other sites, the results suggest that the regulatory power of Ψs and the conditional sensitivity to VPD are characteristic of ET in shrub steppes.

Coupling of carbon dioxide and water vapor exchanges of irrigated and rainfed maize–soybean cropping systems and water productivity

Continuous measurements of CO 2 and water vapor exchanges made in three cropping systems (irrigated continuous maize, irrigated maize–soybean rotation, and rainfed maize–soybean rotation) in eastern Nebraska, USA during 6 years are discussed. Close coupling between seasonal distributions of gross primary production (GPP) and evapotranspiration (ET) were observed in each growing season. Mean growing season totals of GPP in irrigated maize and soybean were 1738 ± 114 and 996 ± 69 g C m −2, respectively (±standard deviation). Corresponding mean values of growing season ET totals were 545 ± 27 and 454 ± 23 mm, respectively. Irrigation affected GPP and ET similarly, both growing season totals were about 10% higher than those of corresponding rainfed crops. Maize, under both irrigated and rainfed conditions, fixed 74% more carbon than soybean while using only 12–20% more water. The green leaf area index (LAI) explained substantial portions (91% for maize and 90% for soybean) of the variability in GPP PAR (GPP over a narrow range of incident photosynthetically active radiation) and in ET/ET o (71% for maize and 75% for soybean, ET o is the reference evapotranspiration). Water productivity (WP or water use efficiency) is defined here as the ratio of cumulative GPP or above-ground biomass and ET (photosynthetic water productivity = ∑GPP/∑ET and biomass water productivity = above-ground biomass/∑ET). When normalized by ET o, the photosynthetic water productivity ( W P E T o ) was 18.4 ± 1.5 g C m −2 for maize and 12.0 ± 1.0 g C m −2 for soybean. When normalized by ET o, the biomass water productivity ( W P E T o ) was 27.5 ± 2.3 g DM m −2 for maize and 14.1 ± 3.1 g DM m −2 for soybean. Comparisons of these results, among different years of measurement and management practices (continuous vs rotation cropping, irrigated vs rainfed) in this study and those from other locations, indicated the conservative nature of normalized water productivity, as also pointed out by previous investigators.

Validation of ET Estimates from the Canadian Prairie Agrometeorological Model for Contrasting Vegetation Types and Growing Seasons

The objective of this study was to establish the ability of the prairie agrometeorological model (PAMII) to simulate daily evapotranspiration (ET). Specifically, modelled ET estimates from PAMII were validated using daily ET estimates from eddy-covariance systems at West Nose Creek (barley field located northwest of Calgary, Alberta) and a FluxNet site (short-grass prairie located west of Lethbridge, Alberta). Additionally, PAMII was validated for three contrasting growing seasons at the grassland site to establish the model’s ability to quantify the effect of different growth conditions on ET. PAMII performed well and was able to capture the day-to-day variability of the ET at both sites. PAMII successfully captured the increase (decrease) in accumulated growing season ET for the wet (dry) growing seasons at the short-grass prairie site. Moreover, the optimal reference minimum stomatal resistance term was significantly lower for the barley crop (50 s m–1) than the corresponding value for the short-grass prairie (80 s m–1). At the grassland site the optimal stomatal resistance term varied markedly depending on the growing conditions; the optimal value for the wet year was 60 s m–1 compared to 90 s m–1 for the dry year. That is, no single value worked best for all years, and our findings caution against using a single value for the reference minimum stomatal resistance. In summary, PAMII captured the salient features of the ET variability at both sites and for contrasting at the grassland site. However, our research has identified several areas where future versions of the PAMII model might be improved.

Canopy Gas Exchange and Water Use Efficiency of ‘Empire’ Apple in Response to Particle Film, Irrigation, and Microclimatic Factors

This study examined the interaction between a reflective particle film and water use efficiency (WUE) response of irrigated and non-irrigated apple trees (Malus ×domestica) over a wide range of environmental conditions. The objectives were to measure isotopic discrimination (Δ13C and δ18O), specific gas exchange, and WUE response of ‘Empire’ apple treated with a reflective particle film (PF), with and without supplemental irrigation, compared with an untreated control, with and without supplemental irrigation, over a range of leaf area indices (LAI), seasonal evapotranspiration (ETo), and vapor pressure deficits (VPD) to determine the mechanisms of action affecting WUE in apple. Short-term whole canopy gas exchange studies and isotope discrimination analysis were used to test the hypothesis that WUE was modified by the use of a PF. In whole canopy gas exchange studies, carbon assimilation (A) and transpiration tended to increase, and WUE and canopy conductance tended to decrease, with VPD within each LAI class from 2 to 6. For VPD &gt; 1 kPa, the PF irrigated treatment consistently had the greatest WUE and other treatments were intermediate for LAI of 2 to 4. The PF irrigated and non-irrigated treatments had greater WUE than the control irrigated and non-irrigated treatments for VPD ≤ 2 kPa and there were no treatment effects for VPD &gt; 2 kPa in the LAI range of 4 to 6. The PF non-irrigated was equivalent to the control non-irrigated treatment at VPD of 1 to 3 kPa, but was significantly lower at VPD of 3 to 4 kPa. PF irrigated and non-irrigated treatments had the greatest carbon isotope discrimination (Δ13C), the control non-irrigated treatment had the lowest Δ13C, and the control-irrigated treatment was intermediate. Oxygen isotope enrichment (δ18O) was positively correlated with the mean growing season VPD and mean growing season evapotranspiration. Δ13C was significantly and positively correlated with δ18O. Seasonal WUE was negatively correlated with Δ13C and there was an interaction with LAI. The seasonal water use of apple is better evaluated with stable isotope discrimination integrating seasonal variation rather that the use of whole canopy gas exchange measurements that measure WUE for brief periods of time. Δ13C was an accurate measurement of apple WUE and indicated that the PF irrigated treatment had the greatest Δ13C and so the lowest WUE compared with the control non-irrigated treatment at LAI from 4 to 6. The reduced WUE of the PF irrigated treatment compared with the control non-irrigated treatment is likely due to increased gS from lower canopy temperature and increased canopy photosynthetically active radiation diffusion that drove increased A. δ18O was an indicator of seasonal water use over six growing seasons due to its high correlation with ETo. In ‘Empire’ apple, A can be increased with PF and irrigation treatments, but at the cost of decreased WUE.

Poplar plantation has the potential to alter the water balance in semiarid Inner Mongolia

Poplar plantation is the most dominant broadleaf forest type in northern China. Since the mid-1990s plantation was intensified to combat desertification along China's northwestern border, i.e., within Inner Mongolia (IM). This evoked much concern regarding the ecological and environmental effects on areas that naturally grow grass or shrub vegetation. To highlight potential consequences of large-scale poplar plantations on the water budget within semiarid IM, we compared the growing season water balance (evapotranspiration (ET) and precipitation (PPT)) of a 3-yr old poplar plantation (Kp 3) and a natural shrubland (Ks) in the Kubuqi Desert in western IM, and a 6-yr old poplar plantation (Bp 6) growing under sub-humid climate near Beijing. The results showed that, despite 33% lower PPT at Kp 3, ET was 2% higher at Kp 3 (228 mm) as compared with Ks (223 mm) in May–September 2006. The difference derived mainly from higher ET at the plantation during drier periods of the growing season, which also indicated that the poplars must have partly transpired groundwater. Estimated growing season ET at Bp 6 was about 550 mm and more than 100% higher than at Kp 3. It is estimated that increases in leaf area index and net radiation at Kp 3 provide future potential for the poplars in Kubuqi to exceed the present ET and ET of the natural shrubland by 100–200%. These increases in ET are only possible through the permanent use of groundwater either directly by the trees or through increased irrigation. This may significantly change the water balance in the area (e.g., high ET at the cost of a reduction in the water table), which renders large-scale plantations a questionable tool in sustainable arid-land management.

Evapotranspiration of irrigated and rainfed maize–soybean cropping systems

We have been making year-round measurements of mass and energy exchange in three cropping systems: (a) irrigated continuous maize, (b) irrigated maize–soybean rotation, and (c) rainfed maize–soybean rotation in eastern Nebraska since 2001. In this paper, we present results on evapotranspiration (ET) of these crops for the first 5 years of our study. Growing season ET in the irrigated and rainfed maize averaged 548 and 482 mm, respectively. In irrigated and rainfed soybean, the average growing season ET was 452 and 431 mm, respectively. On average, the maize ET was higher than the soybean ET by 18% for irrigated crops and by 11% for rainfed crops. The mid-season crop coefficient K c (=ET/ET 0 and ET 0 is the reference ET) for irrigated maize was 1.03 ± 0.07. For rainfed maize, significant dry-down conditions prevailed and mid-season K c was 0.84 ± 0.20. For irrigated soybean, the mid-season K c was 0.98 ± 0.02. The mid-season dry down in rainfed soybean years was not severe and the K c (0.90 ± 0.13) was only slightly lower than the values for the irrigated fields. Non-growing season evaporation ranged from 100 to 172 mm and contributed about 16–28% of the annual ET in irrigated/rainfed maize and 24–26% in irrigated/rainfed soybean. The amount of surface mulch biomass explained 71% of the variability in non-growing season evaporation totals. Water use efficiency (or biomass transpiration efficiency), defined as the ratio of total plant biomass ( Y DM) to growing season transpiration ( T) was 5.20 ± 0.34 and 5.22 ± 0.36 g kg −1, respectively for irrigated and rainfed maize crops. Similarly, the biomass transpiration efficiency for irrigated and rainfed soybean crops was 3.21 ± 0.35 and 2.96 ± 0.30 g kg −1. Thus, the respective biomass transpiration efficiency of these crops was nearly constant regardless of rainfall and irrigation.

Interannual water vapor and energy exchange in an irrigated maize-based agroecosystem

In this paper, we present results from 4 years (May 2001–May 2005) of water and energy flux measurements made in a no-till, irrigated maize–soybean rotation system in eastern Nebraska, USA. The peak green leaf area index (LAI) reached 6.0 and 5.5 in maize (2001 and 2003, respectively) and 5.7 and 4.4 in soybean (2002 and 2004, respectively). The dependence of evapotranspiration (ET) on leaf area was consistent with previous studies. There was a nearly linear relationship between the daily ET/ET o (where ET o is the reference evapotranspiration over a grass reference crop) and LAI until a threshold LAI (between 3 and 4). Above this threshold LAI, the ET/ET o was virtually independent of LAI. The cumulative growing season (planting to harvest) evapotranspiration was 544 and 578 mm for maize, and 474 and 430 mm for soybean. The interannual variability in the growing season ET totals correlated very well with the number of days when the LAI was greater than 3. The non-growing season period (harvest to subsequent planting) contributed between 20 and 25% of the annual ET totals for both crops. The maximum canopy surface conductance ( G smax) was 29 mm s −1 for maize in both years, 41 mm s −1 for soybean in 2002 (peak LAI = 5.7) and 36 mm s −1 for soybean in 2004 (peak LAI = 4.4). The variability in G smax was largely explained by the leaf nitrogen concentration, consistent with the literature.

Design and Performance of Four Evapotranspiration Caps

The protective cap/biobarrier experiment was constructed to compare the performance of three alternative evapotranspiration (ET) caps with that of a U.S. Resource Conservation and Recovery Act recommended cap. One alternative cap configuration was a homogenous soil cap, while two additional alternative cap configurations contained biointrusion barriers placed at different depths within a soil profile. All cap configurations were planted with two vegetation types and were tested under three climate-change precipitation scenarios. Under ambient precipitation, the three alternative cap configurations performed similarly and returned all of the water received from precipitation to the atmosphere during each growing season, regardless of vegetation type. Growing season ET on ambient-precipitation caps ranged from 133 to 338 mm. Native vegetation extracted more water from caps under augmented precipitation regimes than did a grass monoculture. End-of-season soil volumetric water content was 1.7–5.9% lower on caps planted with a native species mix. When planted with native vegetation, the alternative cap designs tested here should easily meet United States Environmental Protection Agency equivalency criteria.

Growing season evapotranspiration from Tamarix ramosissima stands under extreme arid conditions in northwest China

Given the increasing threat of water shortages in the extremely arid regions (rainfall<50 mm year−1) of northwestern China, growing season evapotranspiration (ET) associated with native species such as Tamarix ramosissima is of particular interest. Consequently, this study sought to evaluate the accuracy of daily ET estimates using Bowen ratio data, as well as measuring seasonal ET and development of an energy balance for a stand of T. ramosissima. Bowen ratio apparatus measurements of ET were carried out on a stand of T. ramosissima in a 2.04 km2 field, which are all covered with T. ramosissima near Erdaoqiao, Ejina Province, Inner Mongolia Autonomous Region, China. Soil moisture content and green area index were also monitored. Daily ET could be accurately estimated from Bowen ratio data on a continuous basis over the 153-day growing season. The mean daily T. ramosissima ET over this period was 1.62 mm day−1, for a seasonal total of 248.20 mm. In this extremely arid locale, the great majority of available energy dissipated over the growing season was a result of sensible rather than latent heat fluxes. While the heat storage term was the main source of energy at night and a considerable sink of energy during the daytime, the magnitude of the daily mean soil heat flux was small. Such a determination of temporal and ecosystem-specific ET patterns allows an improved prediction of ET in the desert riparian zone.