Mean Evapotranspiration Rates Research Articles

Evapotranspiration Measurement and Estimation of Crop Coefficient for Native Plant Species of Green Roof in the Tropics

Extensive green roof is one of the sustainable urban stormwater management alternatives to manage and mitigate the urban surface runoff. In order to implement green roofs more effectively, suitable plant species and substrate components for tropical climate must be identified. The aim of this study is to investigate the evapotranspiration (ET) behaviors in extensive green roofs based on different substrate types and local native plant species. Four green roof test beds containing pro-mixing pot and burn soils were each vegetated with Axonopus Compressus (grass) and Portulaca Grandiflora (sedum). A weather station with soil moisture sensors was installed to measure the weather and soil moisture data. The results showed that the mean ET rates for grass-pot soil, sedum-pot soil, grass-burn soil and sedum-burn soil were 1.32 ± 0.41 mm/day, 2.31 ± 0.72 mm/day, 1.47 ± 0.39 mm/day and 2.31 ± 0.43 mm/day, respectively. It is noted that environmental parameters such as ambient temperature, solar radiation and wind speed showed significantly positive relationship (p value < 0.01) with ET rates of green roofs except relative humidity. The crop coefficients (Ks) for the studied green roof plant species are estimated based on actual and reference evapotranspiration rates. The sedum planted in burn soil showed the highest crop coefficient (0.64), followed by sedum in pot soil (0.62), grass in burn soil (0.39) and grass in pot soils (0.37), respectively. The findings in this study also showed that substrate with better water retention capacity generally improved the Ks values.

Identifying the Sources of Continental Summertime Temperature Variance Using a Diagnostic Model of Land–Atmosphere Interactions

AbstractClimate models show that soil moisture and its subseasonal fluctuations have important impacts on the surface latent heat flux, thus regulating surface temperature variations. Using correlations between monthly anomalies in net absorbed radiative fluxes, precipitation, 2-m air temperature, and soil moisture in the ERA-Interim reanalysis and the HadCM3 climate model, we develop a linear diagnostic model to quantify the major effects of land–atmosphere interactions on summertime surface temperature variability. The spatial patterns in 2-m air temperature and soil moisture variance from the diagnostic model are consistent with those from the products from which it was derived, although the diagnostic model generally underpredicts soil moisture variance. We use the diagnostic model to quantify the impact of soil moisture, shortwave radiation, and precipitation anomalies on temperature variance in wet and dry regions. Consistent with other studies, we find that fluctuations in soil moisture amplify temperature variance in dry regions through their impact on latent heat flux, whereas in wet regions temperature variability is muted because of high mean evapotranspiration rates afforded by plentiful surface soil moisture. We demonstrate how the diagnostic model can be used to identify sources of temperature variance bias in climate models.

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.

Evapotranspiration of the Brazilian Pampa Biome: Seasonality and Influential Factors

Experimentally characterizing evapotranspiration (ET) in different biomes around the world is an issue of interest for different areas of science. ET in natural areas of the Brazilian Pampa biome has still not been assessed. In this study, the actual ET (ETact) obtained from eddy covariance measurements over two sites of the Pampa biome was analyzed. The objective was to evaluate the energy partition and seasonal variability of the actual ET of the Pampa biome. Results showed that the latent heat flux was the dominant component in available energy in both the autumn–winter (AW) and spring–summer (SS) periods. Evapotranspiration of the Pampa biome showed strong seasonality, with highest ET rates in the SS period. During the study period, approximately 65% of the net radiation was used for the evapotranspiration process in the Pampa biome. The annual mean ET rate was 2.45 mm d−1. ET did not show to vary significantly between sites, with daily values very similar in both sites. The water availability in the Pampa biome was not a limiting factor for ET, which resulted in a small difference between the reference ET and the actual ET. These results are helpful in achieving a better understanding of the temporal pattern of ET in relation to the landscape of the Pampa biome and its meteorological, soil, and vegetation characteristics.

Hydrological, chemical, and isotopic budgets of Lake Chad: a quantitative assessment of evaporation, transpiration and infiltration fluxes

Abstract. In the Sahelian belt, Lake Chad is a key water body for 13 million people, who live on its resources. It experiences, however, substantial and frequent surface changes. Located at the centre of one of the largest endorheic basins in the world, its waters remain surprisingly fresh. Its low salinity has been attributed to a low infiltration flow whose value remains poorly constrained. Understanding the lake's hydrological behaviour in response to climate variability requires a better constraint of the factors that control its water and chemical balance. Based on the three-pool conceptualization of Lake Chad proposed by Bader et al. (2011), this study aims to quantify the total water outflow from the lake, the respective proportions of evaporation (E), transpiration (T), and infiltration (I), and the associated uncertainties. A Bayesian inversion method based on lake-level data was used, leading to total water loss estimates in each pool (E + T + I = ETI). Sodium and stable isotope mass balances were then used to separate total water losses into E, T, and I components. Despite the scarcity of representative data available on the lake, the combination of these two geochemical tracers is relevant to assess the relative contribution of these three outflows involved in the control of the hydrological budget. Mean evapotranspiration rates were estimated at 2070 ± 100 and 2270 ± 100 mm yr−1 for the southern and northern pools, respectively. Infiltration represents between 100 and 300 mm yr−1 but most of the water is evapotranspirated in the first few kilometres from the shorelines and does not efficiently recharge the Quaternary aquifer. Transpiration is shown to be significant, around 300 mm yr−1 and reaches 500 mm yr−1 in the vegetated zone of the archipelagos. Hydrological and chemical simulations reproduce the marked hydrological change between the normal lake state that occurred before 1972 and the small lake state after 1972 when the lake surface shrunk to a one-tenth of its size. According to our model, shrinking phases are efficient periods for salt evacuation from the lake towards the phreatic aquifer.

Regional evapotranspiration from a wetland in Central Europe, in a 16-year period without human intervention

Regional evapotranspiration (ET) for Fenéki “Pond” (FP, Kis-Balaton wetland, Hungary) was estimated using weighted canopy areas of five macrophyte classes and open water over an uninterrupted period without any human intervention between 1997 and 2012. In addition to the dominant common reed (Phragmites australis), the cover ratio of the other four plant groups and open water was monitored. The different cover categories of the wetland were separated using images with surface interpretation using the maximum likelihood classification method. Although the annual cover ratio for any class only changed slightly by 1–2%, this wetland was not considered to be in equilibrium, since species turnover (sedge-grass) was observed during this study. Reference ET was calculated by the widely accepted Penman–Monteith (FAO-56) formula from the data of a nearby climate station. Of the five macrophyte classes studied, crop coefficients (as the ratio of measured and reference ET) of three groups were measured locally with a lysimeter. To process the 16-year long ET time series, the means of six-season long Kc values were applied. As the open water of FP was frequently covered by seaweed (Ceratophyllumdemersum), a special evaporation (E) was defined using seaweed settled “A” pans. Compared to evaporation of standard “A” pan, the seasonal average evaporation was 7% higher in “A” pans containing seaweed (C.demersum). Annual and seasonal mean ET totals fluctuated depending on environmental conditions (air temperature) and the type of macrophyte. The range of macrophytes’ 16-year annual mean ET varied from 876.8±54.58mm year−1 (dominant common reed) to 662.8±41.57mm year−1 (woody deciduous, Salix fragilis, Alnus glutinosa, Populus tremula) with a weighting average of 802.1±50.4mm year−1 for the regional ET of FP. The average winter ET sum was 17.1±1.19% with a range of 15.7±1.09–20.7±1.43% for common reed and woody deciduous, respectively. The annual mean ET totals were highly correlated with mean daily ET rates of different macrophyte classes. Seasonal daily average ET rates of different macrophyte classes varied between 4.0±0.26mm day−1 and 2.9±0.19mm day−1 in common reed and woody deciduous, respectively. The long-term weighted daily mean ET rate for FP was 3.6±0.24mm day−1. The highest ET/E ratio of 1.09 was for common reed, in an agreement with previous observations, but was lowest in woody deciduous (0.81). In addition to classic statistical approaches, monthly ET data were also used for modeling purposes (ARIMA models). The projected monthly regional ET values supported earlier observed seasonal patterns likely due to peculiarities in wetland water supply. Instead of exclusively using dominant species to estimate regional ET, more accurate results were possible by extending the ET approach to include several macrophyte species. Neglecting the smaller-scale differences in macrophyte, ET could cause more than a 10% overestimation in the regional ET sum. The SARIMA(1,0,0)(0,1,1)12 model contributed positively to the existing information about the ET of the study site. The projected monthly regional ET values retained the earlier observed seasonal pattern until 2020.

Effects of the introduction of rice on evapotranspiration in seasonal wetlands

AbstractLand use changes in wetland areas can alter evapotranspiration, a major component of the water balance, which eventually affects the water cycle and ecosystem. This study assessed the effect of introduced rice‐cropping on evapotranspiration in seasonal wetlands of northern Namibia. By using the Bowen ratio–energy balance method, measurements of evapotranspiration were performed over a period of 2.5 years at two wetland sites—a rice field (RF) and a natural vegetation field (NVF)—and at one upland field (UF) devoid of surface water. The mean evapotranspiration rates of RF (1.9 mm daytime−1) and NVF (1.8 mm daytime−1) were greater than that in UF (1.0 mm daytime−1). RF and NVF showed a slight difference in seasonal variations in evapotranspiration rates. During the dry season, RF evapotranspiration was less than the NVF evapotranspiration. The net radiation in RF was less in this period because of the higher albedo of the non‐vegetated surface after rice harvesting. In the early growth period of rice during the wet season, evapotranspiration in RF was higher than that in NVF, which was attributed to a difference in the evaporation efficiency and the transfer coefficient for latent heat that were both affected by leaf area index (LAI). Evapotranspiration sharply negatively responded to an increase in LAI when surface water is present according to sensitivity analysis, probably because a higher LAI over a surface suppresses evaporation. The control of LAI is therefore a key for reducing evaporation and conserving water. Copyright © 2013 John Wiley & Sons, Ltd.

Land use and limitations in the sinkhole and polje karst of the Ksiromero Region, Western Greece

Ksiromero occupies an area of 107 km2 in the northeast section of the Prefecture of Aitoloakarnania in western Greece. Its karst is developed primarily on beds of 10–200 m thick Triassic carbonate breccia conglomerates which underlie 70 % of the region. Other karstified units overly 200–300 m thick sequences of upper Triassic to early Jurassic limestones and dolomites and an underlying 150 m thick Triassic gypsum deposit. Major karst features include sinkholes and poljes that capture all surface water. Open caves and conduits are rare due to in-fill by residual terra rossa soils. “Ksiromero” is Greek for “dry place.” While in one of the wetter regions of Greece, with a mean annual precipitation of 962–1,040 mm and a mean evapotranspiration rate of 47 %, no perennial springs are known, surface water is rare, and accessible groundwater is minimal. Some shallow wells in the breccia conglomerate serve as small, local water supplies. Geochemical analyses of water samples from two wells show significant differences due to aquifer type (limestone versus terra rossa) but groundwater is suitable at least for irrigation and livestock and probably human consumption. Generally, domestically used water is supplied to Ksiromero from a neighboring region. Water for agriculture is stored in more than 75 reservoirs, some of which are natural sinkholes but most are excavated in terra rossa soils of poljes and large sinkholes. Shallow groundwater occurs in some of these reservoirs, as well as seasonally captured storm water, but much is pumped in from the adjacent region or from nearby wells. This system is adequate for the area’s modest water needs, but not for potential increased demand. Most land use is farming and pasture, and limited to the relatively flat floors of sinkholes and poljes. The absence of sanitary landfills and other agricultural contaminants potentially threaten groundwater quality, but the impacts are not presently quantifiable due to insufficient aquifer characterization.

Estimates of evapotranspiration from MODIS and AMSR-E land surface temperature and moisture over the Southern Great Plains

We apply a new stationarity-based method for estimating the parameters of a simplified land surface model over a 10°×10° study area in the Southern Great Plains. The model is then used to calculate evapotranspiration (ET) using the estimated parameters, atmospheric forcing data (from NLDAS-2) and remotely sensed surface soil moisture (from AMSR-E) and temperature (from AMSR-E and MODIS). By exploiting the statistical relationship between the forcing (e.g., precipitation and radiation) and surface states (soil moisture and temperature), the stationarity-based parameter estimation avoids the restrictive requirements of direct calibration (i.e., the need for ground-truth ET), enabling broad applicability. Modeled annual mean ET rates are compared with benchmark values based on runoff data from Global Runoff Data Center (GRDC), and the root mean square error (RMSE) between them is less than 10Wm−2. Spatial patterns of model estimated ET and of key model parameters indicate that radiation and moisture availability are the controlling factors on ET in the study area. The spatial distributions of the estimated parameters appear reasonable: they are smooth and not overly patchy or random; and they are consistent with the land cover and soil moisture distribution through the area. An important feature of the stationarity-based method demonstrated here is that forcing and surface states need not be continuous. In this application, approximately 30% of the daily surface temperature and moisture data were missing.

Carbon acquisition and water use in a Northern Utah Juniperus osteosperma (Utah juniper) population.

Water use and carbon acquisition were examined in a northern Utah population of Juniperus osteosperma (Torr.) Little. Leaf-level carbon assimilation, which was greatest in the spring and autumn, was limited by soil water availability. Gas exchange, plant water potential and tissue hydrogen stable isotopic ratio (deltaD) data suggested that plants responded rapidly to summer rain events. Based on a leaf area index of 1.4, leaf-level water use and carbon acquisition scaled to canopy-level means of 0.59 mm day(-1) and 0.13 mol m(-2) ground surface day(-1), respectively. Patterns of soil water potential indicated that J. osteosperma dries the soil from the surface downward to a depth of about 1 m. Hydraulic redistribution is a significant process in soil water dynamics. Eddy covariance data indicated a mean evapotranspiration rate of 0.85 mm day(-1) from March to October 2001, during which period the juniper population at the eddy flux site was a net source of CO2 (3.9 mol m(-2) ground area). We discuss these results in relation to the rapid range expansion of juniper species during the past century.

Measurement and analysis of evapotranspiration and surface conductance of a wheat canopy

AbstractThe characteristics of evapotranspiration and canopy surface conductance of winter wheat have been measured and analysed by using the Bowen ratio–energy balance (BREB) method. In order to understand better the effect of soil water status on evapotranspiration and conductance of wheat, the daily and seasonal variations of leaf stomatal resistance under different soil water conditions have also been monitored and analysed. The experiments have been carried out at Luancheng Experimental Station of Agro‐ecosystem (LESA), Heibei, People's Republic of China. During the growing seasons after turning to green, most of the available energy (Rn − G) of the wheat field is partitioned into latent heat. In the daily course, generally, the Bowen ratio (H/LE) for winter wheat is less than one‐third; sometimes, it can reach a value of around 0·4. The seasonal trend of LE/Rn shows a parabola curve. The ratio of LE/Rn varies from 71 to 93%, with the peak occurring in the blooming stage. G/Rn shows a declining logarithmic trend during the observation period, and reaches its minimum value of around 6–8% after blooming. According to the BREB method, the daily mean evapotranspiration rate from jointing to maturity is 3·9 mm day−1, which is very close to the value 4·1 mm derived from the water balance equation. The daily average value for canopy conductance gc is about 7·8 mm s−1. Seasonal trend of leaf stomatal resistance shows that the stomatal resistance in blooming and milking are lower than that in other growing stages. This is possibly determined by the characteristic of water requirement of winter wheat. The stomatal conductance is strongly dependent on soil water status in natural conditions. When the extractable soil water content (ESW) of the 0–40 cm layer is less than 50%, the leaf stomatal conductance gst is linearly dependent on ESW, whereas there is no relation between ESW and gst when ESW is more than 50%. Therefore, the ESW of 50% can be considered as a threshold value of extractable soil water content for controlling the leaf stomatal conductance, or as an indicator of incipient stomatal closure. The slope of the gst–ESW line varies through the growing stages, with the largest slope occurring in the milking stage. Based on the analysis of gst–ESW relations, we construct a simple but realistic model of stomatal conductance–soil moisture relations. Copyright © 2002 John Wiley & Sons, Ltd.

Seasonal variation in the energy and water exchanges above and below a larch forest in eastern Siberia

AbstractThe water and energy exchanges in forests form one of the most important hydro‐meteorological systems. There have been far fewer investigations of the water and heat exchange in high latitude forests than of those in warm, humid regions. There have been few observations of this system in Siberia for an entire growing season, including the snowmelt and leaf‐fall seasons. In this study, the characteristics of the energy and water budgets in an eastern Siberian larch forest were investigated from the snowmelt season to the leaf‐fall season. The latent heat flux was strongly affected by the transpiration activity of the larch trees and increased quickly as the larch stand began to foliate. The sensible heat dropped at that time, although the net all‐wave radiation increased. Consequently, the seasonal variation in the Bowen ratio was clearly ‘U’‐shaped, and the minimum value (1·0) occurred in June and July. The Bowen ratio was very high (10–25) in early spring, just before leaf opening. The canopy resistance for a big leaf model far exceeded the aerodynamic resistance and fluctuated over a much wider range. The canopy resistance was strongly restricted by the saturation deficit, and its minimum value was 100 s m−1 (10 mm s−1 in conductance). This minimum canopy resistance is higher than values obtained for forests in warm, humid regions, but is similar to those measured in other boreal conifer forests. It has been suggested that the senescence of leaves also affects the canopy resistance, which was higher in the leaf‐fall season than in the foliated season. The mean evapotranspiration rate from 21 April 1998 to 7 September 1998 was 1·16 mm day−1, and the maximum rate, 2·9 mm day−1, occurred at the beginning of July. For the growing season from 1 June to 31 August, this rate was 1·5 mm day−1. The total evapotranspiration from the forest (151 mm) exceeded the amount of precipitation (106 mm) and was equal to 73% of the total water input (211 mm), including the snow water equivalent. The understory evapotranspiration reached 35% of the total evapotranspiration, and the interception evaporation was 15% of the gross precipitation. The understory evapotranspiration was high and the interception evaporation was low because the canopy was sparse and the leaf area index was low. Copyright © 2001 John Wiley & Sons, Ltd.

Effect of Crop Geometry (Row Proportions) on the Water Balance of the Root Zone of a Pigeonpea and Rice Intercropping System

SUMMARYRice, pigeonpea and rice + pigeonpea systems (in the row proportions of 1:2 and 2:5) were compared. Soil water depletion and percolation were determined during selected dry spells and yields ascertained after harvest. The mean evapotranspiration rates of rice, pigeonpea, rice + pigeonpea (1:2) and rice + pigeonpea (2:5) were 0.28, 0.79, 0.40 and 0.35 cm d−1, respectively, during a dry spell around 60 days after sowing. In general low rainfall intensity and frequent dry spells in the growing season increased pigeonpea yield but depressed that of rice. Intercropping thus ensured yield stability and hence the profitability of the system as a whole.

Studies on the characteristic features of water consumption in various crops. IV

In order to make clear the characteristics of water consumption of taro plants under upland field conditions, the authors measured both leaf transpiration and evaporation from the soil surface using the chamber method (the vopor-trnsfer method).The measurements were carried out at Taketoyo, Aichi in summer of 1963, 1964 and 1966.1. The amounts of transpiration of taro plants during the growth periods (from late May to early October) were 370mm and 408mm in 1963 and 1964, respectively. For the sameperiod in 1966, the evapotranspiration amount of these plants was 582mm.2. Transpiration rate of taro plants in the early growth stage was little owing to small leaf area, on the other hand the rate of evaporation from the soil surface was much on account of slight covering the soil surface with the foliage of taro plants.The rate of transpiration increased gradually with the increase of leaf area and the rise of air temperature. The maximum transpiration observed was 6.8mm/day on September 10, 1964.About 60% of total amounts of transpiration was transpired for 50 days from July 24 to September 10, 1964. The water requirements were relatively larger (500-800g/g) in the early and late growth stages than the middle growth stage (200-300g/g).3. The relative transpiration increased gradually with the progress of growth, nemely, the increase of leaf area index. This relationship is approximately expressed with the rectangular hyperbolic equation:RT=bL/(1+aL)where RT=relative transpiration, i.e. the value of the rate of transpiration (mm/day) divided by the pan-evaporation (mm/day); L=leaf area index; a and b=constants. The value of inflection point of the hyperbolic curve was about 2.5 of leaf area index, which corresponded to the stage that the soil surface was covered almost thoroughly with the foliage of taro plants.4. Unit relative transpiration, namely, the rate of transpiration (g dm-2 12 hrs-1) divided by the pan-evaporation (mm 12 hrs-1) or daylight hours mean saturation deficit (mm Hg), decreased exponentially with increasing leaf area index. It was approximetely applicable to the above relation that the exponential eqution was given byTp=Tp0e-KLwhere Tp=transpiration power, i.e. unit relative transpiration; Tp0=hypothetical transpiration power when leaf area index was zero; e=base of natural logarithm; L=leaf area index; K=constant.5. The rate of evaporation from the soil surface varied by the covering degrees of the soil surface with the foliage of taro plants.The ratio of the evaporation from the siol surface to the evapotranspiration was about 30% in the stage of largest leaf area index (4.0).6. The largest evapotranspiration rate of taro plants was 9.8mm/day in summer. The daily mean evapotranspiration rate of taro plants was within the range of 5-7mm/day in summer, at Taketoyo, which was estimated by relative evpotranspiration was 1.12-1.14.7. As regards correlations among transpiration, evapotranspiration and various environmental factors, in general transpiration had a positive correlation with leaf area and evapotranspiration had positive correlations with pan-evaporation or solar radiation.