Average Annual Evapotranspiration Research Articles

Effects of Land Use Dynamics on Hydrological Response of Watershed: a Case Study of Chittar Watershed, Vamanapuram River Basin, Thiruvananthapuram District, Kerala, India

Watersheds in the tropical regions of India are vulnerable to the effects of land use/land cover (LULC) change, transpiration and evaporation on groundwater resources. Understanding these changes is important to delineate the impacts on the hydrological cycle and thus helps to strengthen the efficiency of the water management system. A significant change in LULC and evapotranspiration has been observed in the Chittar watershed over the years. The present study involves the assessment of LULC changes during 1967–2001 and 2001–2015. The MODIS MOD 16 A3 data products were used to study evapotranspiration during the period. The groundwater level data of Central Ground Water Board (CGWB) have been used to study the water level changes from 2001 to 2015. The results of the LULC changes indicate that the natural vegetation is decreased significantly from 1967 to 2015. The aerial coverage of natural vegetation was 54% in 1967 which was reduced to 49% in 2001 and 30% in 2015. The most significant land use category is eucalyptus plantation which is decreased from 1967 (12 km2) to 2001 (9 km2). After the year 2011, eucalyptus plantations are converted into acacia plantations. The average annual evapotranspiration of the study area is 917 mm/year during the years 2001 to 2015. The overall evapotranspiration showed a decreasing trend with marked interannual fluctuations. Coming to the groundwater, the upstream of the study area exhibited shallow groundwater level (6.5 to 7.5). From 2001 to 2009, the groundwater level was significantly fluctuated; however, the fluctuation was minimal after 2009. To conclude, the study revealed that the LULC changes in the Chittar watershed have a pivotal role in the observed changes in evapotranspiration. The results reiterated that the changes in evapotranspiration (ET) have a strong bearing on the groundwater potential of the area.

Simulation of soil water dynamics and water productivity of apple trees in different areas of Shaanxi Province, China

Using WinEPIC and partial least squares regression model, we compared the driving factors of water productivity and soil moisture dynamics of several mature apple plantations in Shaanxi from 1981 to 2016. For the hilly and gully region of northern Shaanxi, the residual loess platform region in Weibei, and Guanzhong Plain, the average annual yield of mature apple forests was 16.94, 22.62 and 25.70 t·hm-2, the annual average evapotranspiration was 511.2, 614.9 and 889.88 mm, and the water productivity was 3.81, 3.82 and 3.24 kg·m-3, respectively. In northern Shaanxi and Weibei regions, water stress was the most serious, with the average annual stress days being 54.89 and 28.38 d, respectively. The N-level stress in Guanzhong region was severe, with an average annual stress day of 25.87 d. The largest factor affecting the yield of apple plantations in the northern Shaanxi and northern Weibei regions was the precipitation. The standardized regression coefficients were 0.274 and 0.235, respectively, the amount of N applied had a significant impact on the yield, with regression coefficients of 0.224 and 0.232, respectively. The maximum impact factor in Guanzhong region was the amount of N applied, with a regression coefficient of 0.335, followed by the amount of water supplied and the amount of applied P. The regression coefficients were 0.154 and 0.147, respectively. The dominant factor affecting the water productivity of apple plantations in northern Shaanxi and Weibei was precipitation, and the standardized regression coefficients were 0.238 and 0.194, respectively. The most important impact factors in Guanzhong region were the amount of N applied and the amount of water supplied, and the regression coefficients were 0.182 and 0.178, respectively. During the simulation period, the total water consumption of apple plantations in the northern Shaanxi, Weibei and Guanzhong regions was 1152.17, 1342.95 and 1372.42 mm, respectively. The effective water content decline rates of 2-15 m soil layers were 63.44, 57.08 and 51.41 mm·a-1, respectively. The dry layer of deep soil appeared after 8, 13 and 17 years, and the dry layer was stable to 11 m deep for 18, 21 and 26 years, respectively, suggesting the drying status was severe. The management focus of apple plantations in different regions should be determined by the dominant factors of water productivity.

Comparison of Hydrology of Two Atlantic Coastal Plain Forests

Abstract. This article compares the short-term and long-term hydrology of two typical forests in the humid Atlantic Coastal Plain, including a relatively undisturbed forest with natural drainage in South Carolina (SC) and a drained pine plantation in North Carolina (NC), using monitoring and modeling approaches. Highly dynamic outflow (O) from both of these systems is driven by the water table (WT) position, as influenced by rainfall (R) and evapotranspiration (ET). The annual runoff coefficient (ROC) varied from 5% in dry years to 56% in wet years, depending on the soil water storage (SWS), with a significantly higher average value for the NC site despite its deeper WT, on average, than the SC site. Although both sites behaved similarly in extreme climate conditions, the change in SWS above the WT influenced the annual RO, ROC, and ET. The 17-year average annual ET of 1114 mm (R – O, assuming annual balanced SWS) for the SC site was significantly higher (p = 0.014) than the ET of the drained NC site (997 mm) despite the SC site’s lower mean annual R of 1370 mm, compared to 1520 mm for the NC site. This may be due to both the higher potential ET (PET) and soil water-holding capacity of the SC site. The SC site had higher frequency and duration of WT near the surface during winter, deeper summer WT, and higher correlation of annual ET to annual R (r2 = 0.90 vs. 0.15), suggesting that the SC site was often moisture-limited, particularly during the growing season. Most of the streamflow in these systems occurred during winter, with low ET demands. However, summer periods with tropical storms also resulted in large RO events, generally with higher frequency and longer durations at the drained NC site. These results are similar to an earlier short-term comparison with an unstable behavior period at the SC site after Hurricane Hugo (1989). This study highlighted (1) the differences in hydrology between coastal forests drained for silvicultural production and undrained natural forests managed only for restoration, (2) the importance of long-term monitoring and the effects of regeneration as well as vegetation management on flow regime, and (3) the application and limitations of two widely used models (MIKESHE and DRAINMOD) in describing the hydrology of these forests. Long-term studies can be a basis for testing new hypotheses on water yield, stormwater management, wetland hydrology, vegetation restoration, bioenergy production, and climate change, in addition to applications of proper models for assessing the eco-hydrologic impacts of land use and climate change on freshwater coastal forests linked with downstream riparian rivers and estuaries affected by tidal fluxes and sea level rise.HighlightsOutflow, driven by water table position on these forest systems, is highly variable, depending on its soil water storage.The hydrologic responses of both forest sites were similar during extreme climatic events or disturbances.Effect of forestry drainage on runoff was obscured by its large interannual differences.Long-term monitoring provides better insights on climate and vegetation management effects on flow regime and model validation Keywords: Drainage, Evapotranspiration, Hydrologic models, Pine forest, Poorly drained soils, Runoff coefficient, Water table.

Analysis of Reference Evapotranspiration Change in Korea by Climate Change Impact

In this study, the impact of climate change on the characteristics of evapotranspiration in Korea was assessed. The Regional Concentration Pathway (RCP) 4.5 and 8.5 scenarios were used to calculate the reference evapotranspiration for 62 stations of the Korea Meteorological Administration (KMA) and compared with the calculation results from observation data. The results of the analysis showed that the average annual evapotranspiration increases by 5.4% in the RCP 4.5 scenario and 9.5% in the RCP 8.5 scenario compared with the current one. On a monthly scale, the reference evapotranspiration for January, February, November and December is expected to increase by more than 10% compared to the current period. The ratio of annual rainfall to yearly evapotranspiration was used to examine the possibility of droughts for future periods, and the results were found to be relatively high in the RCP 8.5 scenario when compared to the RCP 4.5 scenario. The increase in the reference evapotranspiration due to climate change may lead to a decrease in the available water resources, and therefore, continuous monitoring and appropriate adaptation measures are necessary. Keywords: Reference Evapotranspiration, Climate Change, RCP Scenario, Hargreaves Equation

Assessing the spatiotemporal distributions of evapotranspiration in the Three Gorges Reservoir Region of China using remote sensing data

Evapotranspiration (ET) is a critical component of the global hydrological cycle, and it has a large impact on water resource management as it affects the availability of freshwater resources. It is important to understand the hydrological cycle for the water resources planning and management. This study used Moderate Resolution Imaging Spectroradiometer (MODIS) satellite derived ET, and potential evapotranspiration (PET) and Tropical Rainfall Measuring Mission (TRMM) satellite derived precipitation datasets to assess the spatial and temporal distributions of ET, PET, and precipitation during the study period at Three Gorges Reservoir (TGR) region. Based on the topographic variations and land-use/land-cover distributions, the study region which includes five counties of Hubei Province and nineteen counties of Chongqing Municipality was divided into four study zones. The ET and precipitation data were evaluated using in situ observations. The ET, PET, and precipitation data were compared to analyze the spatial and long-term (2001-2016) temporal distributions of average annual ET, PET, and precipitation, and to understand the relationships between them in the study region. The results showed that each selected zone had highest ET at the counties with the Yangtze River passing through whereas lowest at the counties which were located away from the river. Results also showed increasing trends in ET and PET from south-west to north-east in the study region. Analysis showed TGR had a significant impact on spatial and temporal distributions of ET and PET in the study region. Therefore, this study helps to understand the impact of TGR on spatial and temporal distributions of ET and PET during and after the construction.

Carbon dioxide, water vapour and energy fluxes over a semi-evergreen forest in Assam, Northeast India

The eddy covariance method is a powerful technique for quantification of \(\hbox {CO}_{2},\) \(\hbox {H}_{2}\)O and energy fluxes in natural ecosystems. Leaf area index (LAI) and its changes are significant drivers of \(\hbox {CO}_{2}\) and \(\hbox {H}_{2}\)O exchange in a forest ecosystem due to their role in photosynthesis. The present study reports the seasonal variation of \(\hbox {CO}_{2}\) and energy fluxes and their relationship with other meteorological parameters of a semi-evergreen primary forest of Kaziranga National Park, Assam, India during February 2016–January 2017. The diurnal pattern of half hourly average \(\hbox {CO}_{2 }\) fluxes over the forest was found to be mostly dominated by the incident photosynthetically active radiation. During the period of study, diurnal variations of \(\hbox {CO}_{2}\) flux showed a maximum value of \(-9.97\,\upmu \)mol \(\hbox {m}^{-2}\hbox {s}^{-1}\) in the month of June during summer which is also the beginning of the monsoon season. The monthly averaged diurnal \(\hbox {CO}_{2}\) flux and variation in LAI of the forest canopy closely followed each other. The annual net ecosystem exchange of the forest estimated from the \(\hbox {CO}_{2}\) flux data above the canopy is 84.21 g C \(\hbox {m}^{-2}\,\hbox {yr}^{-1}\). Further studies are in progress to confirm these findings. The estimated average annual evapotranspiration of the semi-evergreen forest is 2.8 ± 0.19 mm \(\hbox {day}^{-1}\). The study of partitioning of energy fluxes showed the dominance of latent heat fluxes over sensible heat fluxes. The energy balance closure was found to increase with an increase in instability and the highest closure of around 83% was noted under neutral conditions.

Spatial hydrological responses to land use and land cover changes in a typical catchment of the Yangtze River Delta region

This study assessed the individual and combined effects of land-use and land-cover changes (LUCCs) on spatial hydrological responses by using an integrated approach involving the Soil and Water Assessment Tool (SWAT) and geographically weighted regression (GWR) models in the Xitiaoxi River Basin (XRB). The LUCCs and their spatial patterns from 1985 to 2008 were evaluated in the XRB. The hydrological processes during the period from 1980–2015 were then generated by the SWAT model under the 1985 and 2008 land-use scenarios. GWR models were constructed to quantify the spatial impacts of LUCCs at the sub-basin scale. The results showed that the predominant trend of land-use conversion was between forest-grass land and agricultural land, and the diminishing portion of forest-grass land (25.93 km2) and agricultural land (46.77 km2) contributed to the expansion of urban land during the period 1985–2008. Moreover, the urban area increased from 5.6% to 17.05%, and the change ratio progressed towards the centre of the XRB. These changes in land use caused the average annual water yield and surface runoff to increase by 1.09% and 11.87%, respectively, and the average annual evapotranspiration (ET), percolation and baseflow to decrease by 0.73%, 16.53% and 17.26%, respectively. In addition, the results of the GWR models indicated that the responses of each hydrological component exhibited spatial differences. The comprehensive land-use intensification index (La), selected to reflect the combined effects of LUCCs, was positively correlated with water yield and surface runoff but negatively correlated with ET, percolation and baseflow. La had a greater effect on water yield, surface runoff, percolation and baseflow in rural areas than in the urbanized region. The combined effects of LUCCs in certain local areas increased water yield and surface runoff by up to 13.7% and 114.2%, respectively. The local coefficient indicated that rural regions might be exposed to greater effects of LUCCs. The results of this study could be useful to understand the effects of LUCCs on the spatial patterns of each hydrological component and to help improve flood control and water resource management.

Improved vegetation parameterization for hydrological model and assessment of land cover change impacts on flow regime of the Upper Bhima basin, India

This study investigates the potential and applicability of variable infiltration capacity (VIC) hydrological model to simulate different hydrological components of the Upper Bhima basin under two different Land Use Land Cover (LULC) (the year 2000 and 2010) conditions. The total drainage area of the basin was discretized into 1694 grids of about 5.5 km by 5.5 km: accordingly the model parameters were calibrated at each grid level. Vegetation parameters for the model were prepared using temporal profile of Leaf Area Index (LAI) from Moderate-Resolution Imaging Spectroradiometer and LULC. This practice provides a methodological framework for the improved vegetation parameterization along with region-specific condition for the model simulation. The calibrated and validated model was run using the two LULC conditions separately with the same observed meteorological forcing (1996–2001) and soil data. The change in LULC has resulted to an increase in the average annual evapotranspiration over the basin by 7.8%, while the average annual surface runoff and baseflow decreased by 18.86 and 5.83%, respectively. The variability in hydrological components and the spatial variation of each component attributed to LULC were assessed at the basin grid level. It was observed that 80% of the basin grids showed an increase in evapotranspiration (ET) (maximum of 292 mm). While the majority of the grids showed a decrease in surface runoff and baseflow, some of the grids showed an increase (i.e. 21 and 15% of total grids—surface runoff and baseflow, respectively).

Variability, tendencies, and climate controls of terrestrial evapotranspiration and gross primary productivity in the recent decade over China

AbstractAccurately estimating regional water and vegetation carbon fixation and understanding their covariation mechanisms will benefit regional water resources and ecosystem management. The process‐based vegetation interface processes model was employed to simulate the spatiotemporal variations of evapotranspiration (ET) and vegetation gross primary production (GPP) over the land mass of China, by integrating the 8‐day Terra‐MODIS leaf area index dataset from 2000 to 2013. It is found that there are remarkable spatial variations in annual ET and GPP across the country, top values being around 1,200 mm for ET and 3,000 gC m−2 for GPP mostly occurred in the southern rainforests. Average annual ET and GPP are weakly increasing, in which about one third is significant. At annual scale, variability of ET follows precipitation variations, whereas variability of GPP follows variations of both precipitation and leaf area index. At monthly scale, variations of ET and GPP are more correlated with net radiation than precipitation. It is revealed that the climatic factors dominating water and carbon fluxes are different over the typical climate zones. The increasing GPP and water use efficiency are benefitting the regional vegetation recovery and carbon fixation, however, more water consumption may exaggerate the ecosystem vulnerability in water‐limited zones.

Incorporating the Vadose Zone into the Budyko Framework

The Budyko framework provides a quantitative description of long-term average annual evapotranspiration at catchment scales in terms of macro-climatic variables. This framework, however, makes no reference to the vadose zone because it neglects changes in ubsurface storage in the catchment water balance. Recent studies have shown clearly that vadose-zone water storage cannot be neglected at sub-catchment or sub-annual space and time scales, resulting in numerous model attempts to extend the original Budyko framework to incorporate the full water balance equation. Here we apply the approach taken in a companion paper on the original Budyko framework to show that it can be generalized rigorously to include changes in vadose-zone water storage in a manner that is both parsimonious in hypotheses and broad in scope.

Hydrologic cost-effectiveness ratio favors switchgrass production on marginal croplands over existing grasslands.

Switchgrass (Panicum virgatum L.) has attracted attention as a promising second generation biofuel feedstock. Both existing grasslands and marginal croplands have been suggested as targets for conversion to switchgrass, but the resulting production potentials and hydrologic impacts are not clear. The objectives of this study were to model switchgrass production on existing grasslands (scenario-I) and on marginal croplands that have severe to very severe limitations for crop production (scenario-II) and to evaluate the effects on evapotranspiration (ET) and streamflow. The Soil and Water Assessment Tool (SWAT) was applied to the 1063 km2 Skeleton Creek watershed in north-central Oklahoma, a watershed dominated by grasslands (35%) and winter wheat cropland (47%). The simulated average annual yield (2002–2011) for rainfed Alamo switchgrass for both scenarios was 12 Mg ha-1. Yield varied spatially under scenario-I from 6.1 to 15.3 Mg ha-1, while under scenario-II the range was from 8.2 to 13.8 Mg ha-1. Comparison of average annual ET and streamflow between the baseline simulation (existing land use) and scenario-I showed that scenario-I had 5.6% (37 mm) higher average annual ET and 27.7% lower streamflow, representing a 40.7 million m3 yr-1 streamflow reduction. Compared to the baseline, scenario-II had only 0.5% higher ET and 3.2% lower streamflow, but some monthly impacts were larger. In this watershed, the water yield reduction per ton of biomass production (i.e. hydrologic cost-effectiveness ratio) was more than 5X greater under scenario-I than under scenario-II. These results suggest that, from a hydrologic perspective, it may be preferable to convert marginal cropland to switchgrass production rather than converting existing grasslands.

Response of evapotranspiration to changes in land use and land cover and climate in China during 2001–2013

Land surface evapotranspiration (ET) is a central component of the Earth's global energy balance and water cycle. Understanding ET is important in quantifying the impacts of human influences on the hydrological cycle and thus helps improving water use efficiency and strengthening water use planning and watershed management. China has experienced tremendous land use and land cover changes (LUCC) as a result of urbanization and ecological restoration under a broad background of climate change. This study used MODIS data products to analyze how LUCC and climate change affected ET in China in the period 2001–2013. We examined the separate contribution to the estimated ET changes by combining LUCC and climate data. Results showed that the average annual ET in China decreased at a rate of −0.6mm/yr from 2001 to 2013. Areas in which ET decreased significantly were mainly distributed in the northwest China, the central of southwest China, and most regions of south central and east China. The trends of four climatic factors including air temperature, wind speed, sunshine duration, and relative humidity were determined, while the contributions of these four factors to ET were quantified by combining the ET and climate datasets. Among the four climatic factors, sunshine duration and wind speed had the greatest influence on ET. LUCC data from 2001 to 2013 showed that forests, grasslands and croplands in China mutually replaced each other. The reduction of forests had much greater effects on ET than change by other land cover types. Finally, through quantitative separation of the distinct effects of climate change and LUCC on ET, we conclude that climate change was the more significant than LULC change in influencing ET in China during the period 2001–2013. Effective water resource management and vegetation-based ecological restoration efforts in China must consider the effects of climate change on ET and water availability.

Spatio-temporal dynamics of evapotranspiration on the Tibetan Plateau from 2000 to 2010

Evapotranspiration (ET) is a key process of the climate system because it links water, energy and carbon cycles. In this study we modified a Penman–Monteith based algorithm to estimate ET on the Tibetan Plateau at a 1 km spatial resolution for the period 2000–2010 using meteorological and satellite remote sensing data. The results showed that the average annual ET on the Tibetan Plateau was 350.3 mm year−1 and decreased from the southeast toward the northwest. The highest ET value was found in open water bodies (680.9 mm year−1) and the lowest ET value was found in open shrubland (254.0 mm year−1). Overall, the inter-annual ET decreased from 2000 to 2010 and there was significant negative ET trend over 42% of the region, primarily in the northwest of the Tibetan Plateau. Relative humidity was the dominant factor in controlling long-term variations of ET in the arid northwest plateau. But under moist conditions, leaf area index or temperature drove ET. In addition, P-ET on the Tibetan Plateau significantly increased and about 37% of the region showed strong positive P-ET trend primarily in the central of the Tibetan Plateau. The positive P-ET trend in four seasons suggested that the Tibetan Plateau might have become wetter during the past decade.

A multi-model approach for analyzing water balance dynamics in Kathmandu Valley, Nepal

Abstract Study region : Kathmandu Valley, Capital city of Nepal Study focus : This study applied three hydrological models (i.e., SWAT, HBV, and BTOPMC) to analyze the water balance components and their temporal and seasonal variations in the Kathmandu Valley, Nepal. The water balance components were investigated using the same precipitation, climatic data, and potential evapotranspiration (PET) as input variables for each model. The yearly and seasonal variations in each component and the interactions among them were analyzed. There was a close agreement between the monthly observed and calibrated runoff at the watershed scale, and all the three models captured well the flow patterns for most of the seasons. New hydrological insights for the region : The average annual runoff in the study watershed calculated by the SWAT, HBV, and BTOPMC models was 887, 834, and 865 mm, corresponding to 59%, 55%, and 57% of the annual precipitation, respectively. The average annual evapotranspiration (ET) was 625, 623, and 718 mm, and the estimated yearly average total water storage (TWS) was 5, −35, and 29 mm, respectively. The long-term average TWS component was similar in all three models. ET had the lowest inter-annual variation and runoff had the greatest inter-annual variation in all models. Predictive analysis using the three models suggested a reasonable range for estimates of runoff, ET, and TWS. Although there was variation in the estimates among the different models, our results indicate a possible range of variation for those values, which is a useful finding for the short- and long-term planning of water resource development projects in the study area. The effects of historical water use, water stress, and climatic projections using multi-model water balance approaches offer a useful direction for future studies to enhance our understanding of anthropogenic effects in the Kathmandu Valley.

REFERENCE EVAPOTRANSPIRATION (ETO) IN TROPICAL FORESTED AREAS DETERMINED BY PENMAN-MONTEITH MODEL IN CROPWAT SOFTWARE

Reference evapotranspiration (ETₒ) was determined using the Penman-Monteith (P-M) combination equation established in the CROPWAT software v.8 (FAO 1992). The objective of the study is to determine the ETₒ at Bukit Tarek Experimental Watershed, Kerling, Selangor (BTEW), Matang Mangrove at Kuala Sepetang, Perak (MMP) and FRIM Mata Ayer Research Station, Perlis (MAP) using five-year periods of daily weather secondary data. Results showed that ETₒ of 4.10 mm/day at MMP was the highest compared to MAP (3.70 mm/day) and BTEW (3.58 mm/day). The geographical latitude influenced the ETₒ values. The ETo at BTEW was compared with the evapotranspiration (ET) calculated from water balance method which is also at the same location for data period from 1990 to 1994. Results showed that annual average ET was 1552 mm (56.1% of rainfall) while ETo was 43.9% of rainfall only. The annual averages of ET ranged from 3.92 to 4.69 mm day-1 with an average of 4.27 mm day-1. The annual average of ETo was underestimated ET from water balance method ranging from 16.6 to 26.2%. It is expected that the ETo found was underestimated as the actual evapotranspiration requires changes in canopy resistance as function of water availability

Sensitivity of terrestrial water and carbon fluxes to climate variability in semi-humid basins of Haihe River, China

Inter-annual climate variability dominates the evolution of eco-hydrological processes over the water limited basins. Haihe Basin in semi-humid northern China is under significant climate change and serious human disturbance during the recent decades. Integrated with NASA Terra-Modis remote sensing LAI data products, the VIP eco-hydrological model is used to predict the spatiotemporal variations of evapotranspiration (ET) and vegetation gross primary production (GPP) over two catchments of Haihe River branches from 2000 to 2013. Along with precipitation increasing, average annual ET and GPP are significantly increasing. The trends are more distinct over the mountainous upstream areas. The inter-annual variations of ET and GPP are highly correlated with the 12-month SPI (Standardized Precipitation Index), while the monthly GPP is related clearly with 3-month SPI. Sensitivity analysis shows that there are remarkable differences in the elasticity coefficients of both ET and GPP to the climatic factors. It is found that precipitation, solar radiation and leaf area index are the most sensitive factors to ET and GPP in the study basin. The results are helpful to understand the catchment eco-hydrological mechanisms and their responses to climate change.

Variation characteristics and influencing factors of actual evapotranspiration under various vegetation types: A case study in the Huaihe River Basin, China.

The actual evapotranspiration was modelled utilizing the boreal ecosystem productivity simulator (BEPS) in Huaihe River Basin from 2001 to 2012. In the meantime, the quantitative analyses of the spatial-temporal variations of actual evapotranspiration characteristics and its influencing factors under different vegetation types were conducted. The results showed that annual evapotranspiration gradually decreased from southeast to northwest, tended to increase annually, and the monthly change for the average annual evapotranspiration was double-peak curve. The differences of evapotranspiration among vegetation types showed that the farmland was the largest contributor for the evapotranspiration of Huaihe Basin. The annual actual evapotranspiration of the mixed forest per unit area was the largest, and that of the bare ground per unit area was the smallest. The changed average annual evapotranspiration per unit area for various vegetation types indicated an increased tendency other than the bare ground, with a most significant increase trend for the evergreen broadleaf forest. The thermodynamic factors (such as average temperature) were the dominant factors affecting the actual evapotranspiration in the Huaihe Basin, followed by radiation and moisture factors.

Carbon and energy flux from a Phragmites australis wetland in Zhangye oasis-desert area, China

Wetlands play an important role in the exchange of carbon and energy between the land and the atmosphere. Moreover, wetlands are sensitive to global changes because of their unique water-heat effects and greenhouse gas (GHG) metabolic processes. However, the temporal variations in carbon and energy fluxes in wetlands are not yet fully understood. As an artificial wetland in an arid area, the Zhangye wetland features complex meteorological conditions and human interventions, which have introduced uncertainties into the carbon and energy fluxes. In this study, eddy covariance technology was used to examine the characteristics of the carbon and energy fluxes over an artificial wetland in an arid area. The main objectives were to determine (1) the diurnal and seasonal variations in the carbon and energy fluxes; (2) the relationship between the carbon and energy fluxes and the controlling factors, including the meteorological conditions and human interventions; (3) the contribution of carbon emissions from the wetland ecosystem; (4) the evapotranspiration (ET) difference between an artificial wetland in an arid region and a natural wetland; and (5) a preliminary simulation of net ecosystem exchange (NEE) and ET using the Biome-BGC model (Wetland-BGC version). Significant diurnal variations were observed in the carbon dioxide (CO2) flux in different seasons, whereas variations in methane (CH4) were not significant. Both CO2 and CH4 fluxes peaked in summer, with the highest emission rates occurring at 12:00–16:00 and featuring values of −15.65μmolm−2s−1 and 0.38μmolm−2s−1, respectively. The CO2 and CH4 fluxes exhibited a strong relationship with soil temperature (R2=0.7305 and 0.5949, respectively, for a soil depth of 0cm). CH4 emissions significantly influenced the total carbon budget, and the wetland was found to be a carbon sink with respect to the net exchange of carbon. The greatest ET in the Zhangye wetland during the study period was 12.33mmday−1, and the average annual ET was 1300.4mmyear−1. This study examined the main components of the energy flux, including variations in the net radiation (Rn), latent heat flux (LE), sensible heat flux (H) and soil heat flux (Gs), and the relationships between these variables and the environmental controls. The range of LE/Rn was 0.32–0.74, and this ratio was 0.64 during the growing season. The ratio of H/LE ranged from −0.04 to 1.28, and the value was negative during June, July and August. Artificial wetlands had a large thermal capacity that tended to slow down the energy exchange. Human interventions, e.g., irrigation, policies, etc., significantly affected the CH4 flux and ET but did not affect the CO2 flux.

Comparative study of reference evapotranspiration estimation methods including Artificial Neural Network for dry sub-humid agro-ecological region

In the present study, an attempt has been made to compare the reference evapotranspiration (ETo), computed by eight different methods, namely, Penman-Monteith, Modified Penman-Monteith, Hargreaves-Samani, Irmak, Hargreaves, Valiantzas, ANN and FAO(24) model for the dry subhumid agro-ecological region (Varanasi). An attempt was also made to find out utility of artificial neural networks (ANN) for estimation of ET0 with minimum input. Feed forward network has been used for prediction of ETo using resilient back-propagation method and the architecture 2–21(having parameters Tmean and solar radiation) was found to be the best one. The average annual evapotranspiration (by Penman-Monteith method) for Varanasi was found to be 1447.4 mm. When compared among the different methods for estimation of reference evapotranspiration with Penman-Monteith method, the FAO-24 and Hargraves-Samani (3) under estimate, however, Modified Penman-Monteith, Hargreaves-Samani, Irmak, Hargreaves, Valiantzas over-estimate and ANN closely estimates reference evapotranspiration.

A comparison of estimates of basin-scale soil-moisture evapotranspiration and estimates of riparian groundwater evapotranspiration with implications for water budgets in the Verde Valley, Central Arizona, USA

Population growth in the Verde Valley in Arizona has led to efforts to better understand water availability in the watershed. Evapotranspiration (ET) is a critical factor in estimating groundwater recharge in the area and a substantial component of the groundwater budget. In this study, two estimates of soil-moisture ET and two estimates of groundwater ET in the Verde Valley are presented and discussed. Basin-scale soil-moisture potential ET (PET) estimates from the soil-water balance (SWB) and basin characteristics model (BCM) groundwater recharge models are compared. Separately, riparian groundwater ET estimated from a method that uses MODIS-EVI remote sensing data and geospatial information, and from the MODFLOW-EVT ET package as part of a regional groundwater-flow model that includes the study area, are also discussed. Somewhat higher PET rates from the SWB recharge model resulted in an average annual ET volume about 17% greater than for PET from the BCM recharge model. For groundwater ET estimates, annual ET volumes were about the same for upper-bound MODIS-EVI ET for perennial reaches of streams as for the MODFLOW ET estimates, with the small differences between the two methods having minimal impact on annual or longer groundwater budgets for the study area.