Total Evapotranspiration Research Articles

Multiple year effects of a biological control agent (Diorhabda carinulata) on Tamarix (saltcedar) ecosystem exchanges of carbon dioxide and water

Biological control of Tamarix spp. (saltcedar) with Diorhabda carinulata (the northern tamarisk beetle) is currently underway in several western states U.S.A. through historical releases and the natural migration of this insect. Given the widespread dispersal of this biological control agent and its many unknown consequences, this study examines a variety of ecohydrological effects of the beetle on a Tamarix invaded ecosystem in the Great Basin Desert, Nevada. Nearly four years of ecosystem carbon dioxide (CO2) and evapotranspiration (ET) fluxes, measured with an eddy covariance system, are examined in relation to normalized difference vegetation index (NDVI) from Landsat imagery and on the ground measures of leaf area index (LAI) with a light attenuation instrument. We predicted that successive years of beetle herbivory should result in a trajectory of reduced ET and reduced CO2 uptake. We found that three and a half years of beetle herbivory and the resulting defoliation events produced short-term decreases in ET and C uptake. However, total ET and C fluxes over multiple growing seasons were not affected in a clear directional trajectory of reduced ET loss and reduced CO2 uptake, perhaps due to variability in beetle density. LAI and NDVI were fairly well correlated with each other and NDVI was correlated with ET during the summer months, indicating that these measures are useful for detecting beetle damage to the canopy.

Dynamics of evapotranspiration partitioning in a semi-arid forest as affected by temporal rainfall patterns

We extend our recent study of the effects of tree density on evapotranspiration (ET) partitioning in a semi-arid pine forest by examining the influence of the temporal patterns in rainfall (P) on the dynamic contributions of tree transpiration (Tt), soil evaporation (Es) and rainfall interception (IP) to total ET. Soil evaporation accounted for 39% of average annual ET over the four-year period, and was associated with soil moisture content in the upper 5cm and solar radiation, therefore peaking during the wetting and drying seasons (up to 0.75mmday−1). In the dry summer, Es diminished and as much as 50% of the residual flux was due to re-evaporation of moisture condensed at night (adsorption). Tree transpiration accounted for 49% of average annual ET, and was associated with soil moisture at a depth of 10–20cm. Transpiration peaked only in late spring (1.5mmday−1), after the accumulation of large storms allowing infiltration below the topsoil. Moisture at these depths was maintained for longer periods and was even carried over between rain seasons following a high precipitation year. Interception was 12% of annual ET but was larger than 20% during the rainy period. The results indicated that both Tt/ET and Es/ET could vary between 30% and 60% due to their differential response to seasonal environmental drivers. Annual Tt/ET, a major parameter indicating forest productivity and survival, was more influenced by the occurrence of large storms (>30mm; P30/P ratio) than by P itself. In an assessment of the potential warming and drying trends predicted for the Mediterranean region in the next century, changes in both total precipitation and in its temporal patterns must be considered.

Evaluating water scarcity in the Southern African Development Community (SADC) region by using a climate moisture index (CMI) indicator

Increasing water scarcity in the Southern African Development Community (SADC) region has underscored the need to improve our understanding of the management of water resources. Using total evapotranspiration (TET) and precipitation (P) data of the past 16 years this study used a modified version of the climate moisture index (CMI) in order to evaluate water scarcity throughout the SADC region, by examining the relative importance of P and TET on the variability of the CMI. The CMI value for the Democratic Republic of Congo (CMIDRC = 0.347) as well as for Angola (CMIAngola = 0.351), ranged between 0.25 and 1, characteristic of a humid region, whereas CMI values for Botswana (CMIBotswana = 0.027) and South Africa (CMISouth Africa = 0.075) ranged between −0.6 and 0 or 0 and 0.25, characteristic of semi-arid to sub-humid regions. Namibia (CMINamibia = −0.125) has been experiencing drier conditions. The findings of this linear correlation analysis confirm a strong and significant relationship between DRC-Angola (r = 0.837), and a weak but significant relationship between Botswana–Namibia (r = 0.554) and South Africa–Namibia (r = 0.445) with regard to CMI, and suggest the possibility of transferring water from wetter to drier regions in the SADC study area.

Potential natural vegetation dynamics driven by future long-term climate change and its hydrological impacts in the Hanjiang River basin, China

Vegetation and land-surface hydrology are intrinsically linked under long-term climate change. This paper aims to evaluate the dynamics of potential natural vegetation arising from 21st century climate change and its possible impact on the water budget of the Hanjiang River basin in China. Based on predictions of the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios (IPCC-SRES) A1 scenario from the PRECIS (Providing Regional Climates for Impact Studies) regional climate model, changes in plant functional types (PFTs) and leaf area index (LAI) were simulated via the Lund-Potsdam-Jena dynamic global vegetation model. Subsequently, predicted PFTs and LAIs were employed in the Xinanjiang vegetation-hydrology model for rainfall–runoff simulations. Results reveal that future long-term changes in precipitation, air temperature and atmospheric CO2 concentration would remarkably affect the spatiotemporal distribution of PFTs and LAIs. These climate-driven vegetation changes would further influence regional water balance. With the decrease in forest cover in the 21st century, plant transpiration and evaporative loss of intercepted canopy water will tend to fall while soil evaporation may rise considerably. As a result, total evapotranspiration may increase moderately with a slight increase in annual runoff depth. This indicates that, for long-term hydrological prediction, climate-induced changes in terrestrial vegetation cannot be neglected as the terrestrial biosphere plays an important role in land-surface hydrological responses.

Determination of evaporation, transpiration and deep percolation of summer corn and winter wheat after irrigation

The flux of evaporation, transpiration and deep percolation play an important role in agricultural water management. In this study, oxygen-18 was used to determine the three fluxes in the summer corn and winter wheat field under existing irrigation pattern in Shanxi Province, China. Precipitation, irrigation water, soil water, groundwater and stem water were sampled for oxygen-18 analyses, and supported by hydrological observations. By the method of soil water balance and isotope mass balance, combined with eddy correlation method, the following results are reached. After the irrigation on August 11th, 2008 for summer corn (flowering stage, 90 mm, flood irrigation), transpiration of corn accounts for 71.3% of total evapotranspiration, and the irrigation water use efficiency is 38.0%. And after the irrigation on March 15th, 2009 for winter wheat (re-green stage, 110 mm, sprinkler irrigation), transpiration of winter wheat accounts for 61.7% of evapotranspiration, and the irrigation water use efficiency is 42.3%. Compared to flood irrigation, the deep percolation loss of irrigation water under sprinkler irrigation is lower, especially in the first day after irrigation. Overall, the existing irrigation efficiency is low in study area, and measures should be taken to reduce the deep percolation after irrigation.

Alternative methods to predict actual evapotranspiration illustrate the importance of accounting for phenology – Part 2: The event driven phenology model

Abstract. Evapotranspiration (ET) flux constitutes a major component of both the water and energy balances at the land surface. Among the many factors that control evapotranspiration, phenology poses a major source of uncertainty in attempts to predict ET. Contemporary approaches to ET modeling and monitoring frequently summarize the complexity of the seasonal development of vegetation cover into static phenological trajectories (or climatologies) that lack sensitivity to changing environmental conditions. The Event Driven Phenology Model (EDPM) offers an alternative, interactive approach to representing phenology. This study presents the results of an experiment designed to illustrate the differences in ET arising from various techniques used to mimic phenology in models of land surface processes. The experiment compares and contrasts two realizations of static phenologies derived from long-term satellite observations of the Normalized Difference Vegetation Index (NDVI) against canopy trajectories produced by the interactive EDPM trained on flux tower observations. The assessment was carried out through validation of predicted ET against records collected by flux tower instruments. The VegET model (Senay, 2008) was used as a framework to estimate daily actual evapotranspiration and supplied with seasonal canopy trajectories produced by the EDPM and traditional techniques. The interactive approach presented the following advantages over phenology modeled with static climatologies: (a) lower prediction bias in crops; (b) smaller root mean square error in daily ET – 0.5 mm per day on average; (c) stable level of errors throughout the season similar among different land cover types and locations; and (d) better estimation of season duration and total seasonal ET.

Yield and water productivity of Bt cotton (Gossypium hirsutum) as influenced by temperature under semi-arid conditions of north-western India: field and simulation study

Cotton cultivation in semi-arid region of Indian Punjab is considered as most risky crop as its yield is very sensitive to weather parameters like rain and temperature. In future due to global warming increase in the temperature is expected, which is most likely to influence the growth and yield of this fiber crop like other cereal crops. Studies on the effect of temperature on the growth and seed yield of Bt cotton in this region are lacking. The present 2-year field and 15-year simulation studies concern to simulate the effect of temperature on duration of pheno-phases and seed yield of Bt cotton hybrid RCH 134 and also on crop water productivity. Simulations were run for 15 years (1991–2005) using the already customized CropSyst model. The simulated results indicated that with increase in temperature from 28 to 32oC, cotton seed yield was reduced to half (from 4 700 to 2 300 kg/ha) following a linear relation with high coefficient of determination (0.97), and the reduction was more with increased temperature during sowing to flowering stage than other pheno-phases. Total evapo-transpiration (ET) during crop period and crop water productivity was also decreased with increased temperature. Relationship of cotton seed yield was linear with ET and quadratic with total water supply (rain +irrigation). Real crop water productivity (yield/ET) and apparent crop water productivity (yield/irrigation water) were 0.362+0.129 and 0.485+0.120 kg/m3, respectively.

Subpixel heterogeneity of ice-wedge polygonal tundra: a multi-scale analysis of land cover and evapotranspiration in the Lena River Delta, Siberia

ABSTRACTIgnoring small-scale heterogeneities in Arctic land cover may bias estimates of water, heat and carbon fluxes in large-scale climate and ecosystem models. We investigated subpixel-scale heterogeneity in CHRIS/PROBA and Landsat-7 ETM+ satellite imagery over ice-wedge polygonal tundra in the Lena Delta of Siberia, and the associated implications for evapotranspiration (ET) estimation. Field measurements were combined with aerial and satellite data to link fine-scale (0.3 m resolution) with coarse-scale (up to 30 m resolution) land cover data. A large portion of the total wet tundra (80%) and water body area (30%) appeared in the form of patches less than 0.1 ha in size, which could not be resolved with satellite data. Wet tundra and small water bodies represented about half of the total ET in summer. Their contribution was reduced to 20% in fall, during which ET rates from dry tundra were highest instead. Inclusion of subpixel-scale water bodies increased the total water surface area of the Lena Delta from 13% to 20%. The actual land/water proportions within each composite satellite pixel was best captured with Landsat data using a statistical downscaling approach, which is recommended for reliable large-scale modelling of water, heat and carbon exchange from permafrost landscapes.

Mapping Spatially Interpolated Precipitation, Reference Evapotranspiration, Actual Crop Evapotranspiration, and Net Irrigation Requirements in Nebraska: Part II. Actual Crop Evapotranspiration and Net Irrigation Requirements

With the world population expected to increase by 3.7 billion people by 2050, it is anticipated that the additional food and fiber required to feed future generations will impose greater pressure on freshwater resources. Approximately 70% to 80% of global freshwater withdrawal is for irrigation, and more than 40% of food is produced on irrigated land, which comprises only 18% of the total land allocated to food and fiber production. To evaluate future water supply and demand in relation to food production, it is necessary to quantify actual crop evapotranspiration (ETa) and irrigation water requirements (NIR) of various crop production systems. Long-term (1986-2009) growing season (1 May to 30 September) ETa and NIR for maize and soybean were quantified for all 93 counties in Nebraska. There was a gradual decrease in ETa totals from the western part (zone 1) of the state toward the eastern part (zone 4). For maize, the growing season total ETa ranged from 801 mm in Cheyenne County (zone 1) to 484 mm in Butler County (zone 4), with a statewide average of 634 mm. For soybean, the growing season ETa ranged from 705 mm in Cheyenne County to 430 mm in Butler County, with a statewide average of 559 mm. On a statewide average, maize ETa was 75 mm (12%) higher than soybean ETa. There was a gradual decrease in NIR totals from the western part of the state toward the eastern part. The NIR for maize ranged from 575 mm in the western part to 150 mm in eastern Nebraska, with a statewide average of 289 mm. Soybean NIR ranged from 467 mm in the western part to 111 mm in the eastern counties, with a statewide average of 227 mm. On average, the NIR was greater in zone 1 than in zone 4 by 319 mm (61.7%) for maize and 267 mm (63.04%) for soybean. July was the most critical for both crops in terms of contribution of precipitation to ETa and NIR. The statewide average July NIR was observed as 133 mm and 127 mm for maize and soybean, respectively. The statewide long-term average minimum NIR for maize was observed in 1993 as 90 mm (±88 mm) and in 1996 as 60 mm (±100 mm) for soybean, and the maximum value was obtained in 2002 as 525 mm (±188 mm) for maize and 433 mm (±173 mm) for soybean. In the last 24-year period, the statewide average maize NIR increased by 21 mm, and the average soybean NIR increased by 18 mm. The long-term average statewide NIR was 268 mm (±154 mm) for maize and 196 mm (±137 mm) for soybean. Mapping and quantification of ETa and NIR can help water users and managers, decision makers, and policymakers better allocate water resources and develop water resources management strategies for enhancing crop water productivity. These datasets and analyses can also aid in identifying locations where long-term trends in ETa, NIR, and crop productivity are decreasing so that more robust resource reallocation practices can be adopted to improve agricultural productivity.

A method to calculate heterogeneous evapotranspiration using submeter thermal infrared imagery coupled to a stomatal resistance submodel

Thermal infrared (TIR) remote sensing of vegetation temperature, combined with surface energy balance modeling, allows efficient estimation of spatially distributed evapotranspiration (ET). Many ET models are sensitive to the parameterization of stomatal control; yet, modelers often employ spatially uniform stomatal resistance values, even in distributed applications. Unfortunately, assuming uniform resistance across a canopy with large temperature variance is physically unrealistic and may produce artifacts in ET magnitude. To account for spatial variations in stomatal control that likely accompany temperature variations, we propose nesting a new submodel within some well‐established ET models. The submodel derives, for the canopy patch of interest, a concave‐downward relationship between stomatal conductance and temperature, as expected from plant biology. Using the submodel, each pixel's contribution to the total canopy patch ET is influenced both by its observed temperature and by its location‐specific estimated stomatal resistance. The submodel requires only one more parameter than the unmodified ET models, which can be obtained from the literature; it conserves energy between the pixel and image scales, unlike single‐valued resistance approaches; it produces realistic ET values at extreme temperature locations; and provides a remote sensing‐based way to estimate the in situ canopy stomatal conductance‐temperature relationship, which otherwise must be measured under controlled conditions. Since very high‐resolution TIR data provide one means to observe large temperature variance, the submodel was tested using data with cm‐scale pixels collected over 1.5 m2 patches of two vegetation types. The biophysical relationships derived by the submodel were successfully verified against laboratory data.

The Impacts of the Interannual Variability of Vegetation on the Interannual Variability of Global Evapotranspiration: A Modeling Study

AbstractThe impact of the interannual variability (IAV) of vegetation on the I AV of evapotranspiration is investigated with the Community Land Model (CLM3.0) and modified Dynamic Global Vegetation Model (DGVM). Two sets of 50-year off-line simulations are used in this study. The simulations begin with the same initial surface-water and heat states and are driven by the same atmospheric forcing data. The vegetation exhibits interannual variability in one simulation but not in the other simulation. However, the climatological means for the vegetation are the same. The IAV of the 50-year annual total evapotranspiration and its three partitions (ground evaporation, canopy evaporation, and transpiration) are analyzed. The global distribution of the evapotranspiration I AV and the statistics of evapotranspiration and its components in different ecosystems show that the IAV of ground evaporation is generally large in areas dominated by grass and deciduous trees, whereas the IAV of canopy evaporation and transpi...

Nonlinear controls on evapotranspiration in arctic coastal wetlands

Abstract. Projected increases in air temperature and precipitation due to climate change in Arctic wetlands could dramatically affect ecosystem function. As a consequence, it is important to define controls on evapotranspiration, the major pathway of water loss from these systems. We quantified the multi-year controls on midday Arctic coastal wetland evapotranspiration, measured with the eddy covariance method at two vegetated, drained thaw lake basins near Barrow, Alaska. Variations in near-surface soil moisture and atmospheric vapor pressure deficits were found to have nonlinear effects on midday evapotranspiration rates. Vapor pressure deficits (VPD) near 0.3 kPa appeared to be an important hydrological threshold, allowing latent heat flux to persistently exceed sensible heat flux. Dry (compared to wet) soils increased bulk surface resistance (water-limited). Wet soils favored ground heat flux and therefore limited the energy available to sensible and latent heat flux (energy-limited). Thus, midday evapotranspiration was suppressed from both dry and wet soils but through different mechanisms. We also found that wet soils (ponding excluded) combined with large VPD, resulted in an increased bulk surface resistance and therefore suppressing evapotranspiration below its potential rate (Priestley-Taylor α < 1.26). This was likely caused by the limited ability of mosses to transfer moisture during large atmospheric demands. Ultimately, in addition to net radiation, the various controlling factors on midday evapotranspiration (i.e., near-surface soil moisture, atmospheric vapor pressure, and the limited ability of saturated mosses to transfer water during high VPD) resulted in an average evapotranspiration rate of up to 75% of the potential evapotranspiration rate. These multiple limitations on midday evapotranspiration rates have the potential to moderate interannual variation of total evapotranspiration and reduce excessive water loss in a warmer climate. Combined with the prevailing maritime winds and projected increases in precipitation, these curbing mechanisms will likely prevent extensive future soil drying and hence maintain the presence of coastal wetlands.

Simple physics-based models of compensatory plant water uptake: concepts and eco-hydrological consequences

Abstract. Many land surface schemes and simulation models of plant growth designed for practical use employ simple empirical sub-models of root water uptake that cannot adequately reflect the critical role water uptake from sparsely rooted deep subsoil plays in meeting atmospheric transpiration demand in water-limited environments, especially in the presence of shallow groundwater. A failure to account for this so-called "compensatory" water uptake may have serious consequences for both local and global modeling of water and energy fluxes, carbon balances and climate. Some purely empirical compensatory root water uptake models have been proposed, but they are of limited use in global modeling exercises since their parameters cannot be related to measurable soil and vegetation properties. A parsimonious physics-based model of uptake compensation has been developed that requires no more parameters than empirical approaches. This model is described and some aspects of its behavior are illustrated with the help of example simulations. These analyses demonstrate that hydraulic lift can be considered as an extreme form of compensation and that the degree of compensation is principally a function of soil capillarity and the ratio of total effective root length to potential transpiration. Thus, uptake compensation increases as root to leaf area ratios increase, since potential transpiration depends on leaf area. Results of "scenario" simulations for two case studies, one at the local scale (riparian vegetation growing above shallow water tables in seasonally dry or arid climates) and one at a global scale (water balances across an aridity gradient in the continental USA), are presented to illustrate biases in model predictions that arise when water uptake compensation is neglected. In the first case, it is shown that only a compensated model can match the strong relationships between water table depth and leaf area and transpiration observed in riparian forest ecosystems, where sparse roots in the capillary fringe contribute a significant proportion of the water uptake during extended dry periods. The results of the second case study suggest that uncompensated models may give biased estimates of long-term evapotranspiration at the continental scale. In the example presented here, the uncompensated model underestimated total evapotranspiration by 5–7% in climates of intermediate aridity, while the ratio of transpiration to evaporation was also smaller than for the compensated model, especially in arid climates. It is concluded that the parsimonious physics-based model concepts described here may be useful in the context of eco-hydrological modeling at local, regional and global scales.

Components of the water balance in soil with sugarcane crops

Abstract The objective of this study was to analyze the components of the water balance in an Ultisol, located in the municipality of Jaboticabal, SP, Brazil (21°20′20″S, 48°18′35″W), that was cultivated with sugarcane. The monitoring was performed during the agricultural cycle of the first ratoon between 11/16/2006 and 7/9/2007. Three treatments were established in four blocks with doses of ammonium sulfate, as follows: Treatment 1 (T1), without fertilizer; Treatment 2 (T2), 100 kg ha−1 of nitrogen (N) and 114 kg ha−1 of sulfur (S); and Treatment 3 (T3), 150 kg ha−1 of N and 172 kg ha−1 of S. Rainy precipitation (P) in the area was measured with a rain gauge. The soil water storage (H) and the soil water storage variations (ΔH) were determined by the gravimetric method, and the internal drainage (D)/capillary rise (CR) at a depth of 0.9 m was quantified by the water flux density using the Darcy–Buckingham equation. The actual evapotranspiration (ETa) was calculated as follows: ETa = P − D + CR ± ΔH. During the study period, the amount of rainfall was 1406 mm, 121 mm greater than the historic average for the region (1285 mm), with a notable peak in the month of January of 402 mm (historic average: 251 mm). The internal drainage was 300 mm under T1, 352 mm under T2, and 199 mm under T3, and this was relevant during times with elevated P, when the actual H was greater than the field capacity H. The actual evapotranspiration (T1: −897.7 mm, T2: −847.5 mm, and T3: −970.8 mm) and the water use efficiency (T1: −131.3 kg mm−1, T2: −146.6 kg mm−1, and T3: −127.5 kg mm−1) did not differ among the treatments. The dispersion of D was greater than the other components of the water balance, especially during the period of elevated P, with the errors of this process propagated in the estimation of ETa. Despite of this propagated standard deviation of ETa, it accounted less than 15% of the total ETa, showing that the method may be conveniently used in field studies with sugarcane crops.

Modelling evapotranspiration in a Central Asian desert ecosystem

In current terrestrial ecosystem modelling used by hydrological and biochemical cycle researchers, arid and semiarid biomes should receive much more attention because they cover a large proportion of total land area. Limited understanding of ecophysiological parameters of desert shrubs makes it very difficult to estimate evapotranspiration with ecosystem models over Central Asia. This research determined two important parameters, the initial quantum efficiency ( α) and the maximum carboxylation rate ( V cmax), required in the current widely used modelling strategies for two desert shrubs ( Haloxylon ammodendron and Tamarix ramosissima). The photosynthetic parameters were applied to a coupled model of stomatal conductance, photosynthesis and transpiration to simulate the diurnal dynamics of transpiration and seasonal patterns of evapotranspiration and soil evaporation. The stomatal conductance, photosynthesis, and transpiration models were adopted from Ball–Berry, Farquhar (for C 3), Collatz (for C 4), and Shuttleworth–Wallace as commonly parameterised in most current terrestrial ecosystem models. It was found that the Shuttleworth–Wallace model was able to explain 61% and 52% of the variances of the measured transpiration for T. ramosissima and H. ammodendron respectively. In addition, the model explained 78% and 57% of the variances of the observed total evapotranspiration and soil evaporation for a desert ecosystem where T. ramosissima is sparsely distributed. This result suggests that the Shuttleworth–Wallace model implemented with a coupled photosynthesis–stomatal conductance–transpiration modelling strategy is a promising approach for estimating evapotranspiration in desert ecosystems over Central Asia. In contrast, a very simple format of evapotranspiration modelling approach, the Priestley–Taylor model, was only able to yield a fair estimation of evapotranspiration during some periods of the growing season even though the soil moisture effect is integrated into the Priestley–Taylor parameter. We recommend that the physically based Shuttleworth–Wallace model be used for estimating evapotranspiration in related ecohydrological research in desert ecosystems over Central Asia.

Afforestation of Tropical Pasture Only Marginally Affects Ecosystem-Scale Evapotranspiration

Evapotranspiration (ET) from tropical ecosystems is a major constituent of the global land–atmosphere water flux and strongly influences the global hydrological cycle. Most previous studies of ecosystem ET have been conducted predominantly in tropical forests, and only few observations cover other tropical land-use types such as pastures, croplands, savannas or plantations. The objectives of our study were: (1) to estimate daily, monthly, and annual ET budgets in a tropical pasture and an adjacent afforestation site, (2) to assess diurnal and seasonal patterns of ET, (3) to investigate environmental controls of ET, and (4) to evaluate the soil infiltration potential. We performed eddy covariance measurements of ecosystem ET in Sardinilla (Panama) from 2007 to 2009. Daily ET (2.6 ± 1.0 mm day−1) was significantly lower in the pasture compared to the afforestation site (3.0 ± 0.9 mm day−1). The highest ET was observed during the wet–dry transition period in both ecosystems. However, differences in daily ET between sites were relatively small, particularly during the wet season. Radiation was the main environmental control of ET at both sites, however, we observed considerable seasonal variation in the strength of this control, which was stronger during the wet compared to the dry season. In 2008, total annual ET was only slightly higher for the afforestation (1114 mm y−1) than the pasture site (1034 mm y−1). Our results suggest that afforestation of pasture only marginally increases ecosystem-scale ET 6–8 years after establishment. Differences in soil infiltration potentials between our sites seem to explain this pattern.

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

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

The influence of trees and grass on outdoor thermal comfort in a hot‐arid environment

AbstractThe effects of vegetation on human thermal stress in a hot‐arid region were tested in two semi‐enclosed urban spaces with various combinations of mature trees, grass, overhead shading mesh and paving. The index of thermal stress was calculated hourly from measured meteorological data in the studied sites to evaluate thermal comfort in the different spaces based on radiative and convective pedestrian–environment energy exchanges and sweat efficiency, and expressed on a thermal sensation scale ranging from ‘comfortable’ to ‘very hot’. The efficiency of water use in providing improved comfort was gauged for each of the vegetative landscaping treatments by comparing the total evapotranspiration with the reduction in thermal stress, both expressed in terms of their values in equivalent energy. While conditions in a paved, unshaded courtyard were found to be uncomfortable throughout the daytime hours (with half of these hours defined by severe discomfort), each of the landscape treatments made a clear contribution to improved thermal comfort. With shading, either by trees or mesh, discomfort was reduced in duration by over half and limited in maximum severity when the shading was placed above paving. When combined with grass, both shading mechanisms yielded comfortable conditions at all hours. In both cases, the effect of trees was more pronounced than that of the mesh, but by a small margin. With unshaded grass, ‘hot’ conditions in the courtyard were restricted to a short period in mid‐afternoon, a considerable improvement over unshaded paving, attributable mainly to the lower radiant surface temperatures. Copyright © 2010 Royal Meteorological Society

The effects of mulch and irrigation management on wheat in Punjab, India—Evaluation of the APSIM model

With increasing interest in retaining crop residues on the soil surface, there is a need to evaluate their short- and long-term effects on crop yield and water and fertilizer requirements. Therefore, research on the interactions between residue and irrigation management on wheat crop performance and water use was initiated, using the dual approach of field experiments and crop modelling. This paper presents the results of a comprehensive evaluation of the APSIM model for its ability to simulate the effects of mulch and water management, and their interactions, for wheat in Punjab, India. The model was evaluated for its ability to predict crop development, grain yield, biomass production over time, soil water dynamics, daily soil evaporation (Es), total evapotranspiration (ET) and water productivity (WP ET kg ha −1 mm −1), using two years of data from field experiments at Ludhiana, Punjab. The model predicted grain yield adequately, with coefficients of determination ( r 2) of 0.91 and 0.81 with and without mulch, respectively, and prediction of total biomass was even better, with r 2 of 0.99 and 0.92. The corresponding absolute RMSE values were 433 and 550 kg ha −1 for grain yield (means 4100 and 3800 kg ha −1) and 300 and 800 kg ha −1 for total biomass (means 10,200 and 9300 kg ha −1). However, grain yield was underpredicted (by 600–1000 kg ha −1) in treatments where the crop was subjected to water deficit stress, even though simulation of soil water dynamics, and the effect mulch on soil water content, was generally very good. The model accurately predicted total crop seasonal evaporation and the effect of mulch; however, daily Es was poorly simulated. APSIM does not attempt to capture the soil temperature driven effect of mulch on crop phenology. The evaluation shows that APSIM is suitable to use for wheat under the conditions of north-west India. However, additional model processes that capture the effects of mulch on crop development and growth, as driven by soil temperature, are needed to help design intensive cropping systems to optimise land and water productivity. The ability to better simulate crop performance under conditions of water deficit is also needed to help determine irrigation management strategies that minimise irrigation input while maintaining yield.

산림에서의 젖은 군락 증발 관측: 고찰

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