Changes In Potential Evapotranspiration Research Articles

50,000 years of vegetation and climate change in the southern Namib Desert, Pella, South Africa

This paper presents the first continuous pollen record from the southern Namib Desert spanning the last 50,000years. Obtained from rock hyrax middens found near the town of Pella, South Africa, these data are used to reconstruct vegetation change and quantitative estimates of temperature and aridity. Results indicate that the last glacial period was characterised by increased water availability at the site relative to the Holocene. Changes in temperature and potential evapotranspiration appear to have played a significant role in determining the hydrologic balance. The record can be considered in two sections: 1) the last glacial period, when low temperatures favoured the development of more mesic Nama-Karoo vegetation at the site, with periods of increased humidity concurrent with increased coastal upwelling, both responding to lower global/regional temperatures; and 2) the Holocene, during which time high temperatures and potential evapotranspiration resulted in increased aridity and an expansion of the Desert Biome. During this latter period, increases in upwelling intensity created drier conditions at the site.Considered in the context of discussions of forcing mechanisms of regional climate change and environmental dynamics, the results from Pella stand in clear contrast with many inferences of terrestrial environmental change derived from regional marine records. Observations of a strong precessional signal and interpretations of increased humidity during phases of high local summer insolation in the marine records are not consistent with the data from Pella. Similarly, while high percentages of Restionaceae pollen has been observed in marine sediments during the last glacial period, they do not exceed 1% of the assemblage from Pella, indicating that no significant expansion of the Fynbos Biome has occurred during the last 50,000years. These findings pose interesting questions regarding the nature of environmental change in southwestern Africa, and the significance of the diverse records that have been obtained from the region.

Potential evapotranspiration change and its attribution in the Qinling Mountains and surrounding area, China, during 1960–2012

Based on the observational data of 47 meteorological stations in the northern and southern regions of the Qinling Mountains (NSQ) during 1960–2012, this paper estimated the potential evapotranspiration (ET0) by using the Penman–Monteith method. Further, a quantitative study was conducted of the ET0 spatial distribution pattern, temporal variation rules, influencing factors and attributions. The conclusions were as follows. (1) The spatial distribution of annual ET0 in NSQ decreased from northeast to southwest. The seasonal distribution was summer > spring > autumn > winter. (2) Further, 1979 and 1993 were the turning points of the ET0 trend, at which the value began to decrease or increase over the whole region and sub-regions. At the seasonal scale, in the period of 1960–1979, ET0 in spring, summer, and winter presented a decreasing trend; however, it increased slightly in autumn. During 1980–1993, ET0 in most seasons showed a downward trend except for autumn; in the period of 1994–2012, ET0 declined in summer and autumn, however it increased slightly in spring and winter. (3) The diurnal temperature range during 1960–1979 contributed most to ET0. The decrease of wind speed and solar radiation were the main cause of the ET0 decrease during 1980–2012, which offset the effect of the increase in temperature.

Karst catchments exhibited higher degradation stress from climate change than the non-karst catchments in southwest China: An ecohydrological perspective

Karst landform represents about 10% of the continental area and plays key roles in water supplies for almost a quarter of the global population. Knowledge of ecohydrological responses of karst landform to climate change is critical for both water resources management and ecological protection in these regions. This study investigated the effects of karst landform on the elasticity of actual evapotranspiration (derived by the Budyko equation), estimated the contribution of climate change and evaluated the implications, on the basis of 13 typical catchments that have different karst landform coverages in southwest China. Catchment properties, including the vegetation coverage, portion of karst landform (POK), drainage area, surface roughness, mean topographic wetness index, mean slope, and mean aspect, were selected to test the influencing factors for the elasticity of actual evapotranspiration. Results indicate that POK is the most influencing factor for the elasticity of actual evapotranspiration in this region. Moreover, the actual evapotranspiration in karst catchments is more sensitive to precipitation change and less sensitive to the potential evapotranspiration change than that in the non-karst catchments. On the other hand, the contribution of climate change to actual evapotranspiration was generally negative in this region. Furthermore, relatively large negative contributions mainly occurred in the karst-dominated catchments, suggesting that the karst catchments were exposed to higher degradation stress brought by the climate change than that in non-karst catchments.

Evolution of potential evapotranspiration in the northern Loess Plateau of China: recent trends and climatic drivers

ABSTRACTPotential evapotranspiration represents the maximum evaporative capacity for a region, which underscores the importance of analysing its changes and attribution to provide information for water resource management. This study focused on the northern Loess Plateau of China, investigated the changes in potential evapotranspiration (ET0) using data from 34 stations during 1960–2013 and conducted its temporal and spatial attribution through the differentiation equation method. The results show that the annual mean ET0 is 1004.9 mm with a decreasing trend at the rate of 0.33 mm year−1. The lowest values were found in the northeast and southwest regions, while the greatest ET0 was in the northwest region. An ‘evaporation paradox’ existed as a whole, but not obviously. The ET0 was most sensitive to actual vapour pressure (ea), followed by solar radiation (Rs), mean temperature (Tmean) and wind speed (U2). The inter‐annual variation in the sensitivity of ET0 in relation to climatic factors showed that ET0 became more sensitive to U2 and Tmean but less sensitive to Rs and ea from 1960 to 2013. Temporally, the increasing Tmean resulted in the increase of ET0; while its effect was weakened by the other three factors, especially by decreasing U2. The contributions of climatic factors to the ET0 spatial variation trends were also analysed along two transects, but the dominant factors were different for each transect. Furthermore, the possible impacts of human activities on ET0 changes were also discussed.

Projecting impacts of climate change on hydrological conditions and biotic responses in a chalk valley riparian wetland

Projected changes in climate are likely to substantially impact wetland hydrological conditions that will in turn have implications for wetland ecology. Assessing ecohydrological impacts of climate change requires models that can accurately simulate water levels at the fine-scale resolution to which species and communities respond. Hydrological conditions within the Lambourn Observatory at Boxford, Berkshire, UK were simulated using the physically based, distributed model MIKE SHE, calibrated to contemporary surface and groundwater levels. The site is a 10 ha lowland riparian wetland where complex geological conditions and channel management exert strong influences on the hydrological regime. Projected changes in precipitation, potential evapotranspiration, channel discharge and groundwater level were derived from the UK Climate Projections 2009 ensemble of climate models for the 2080s under different scenarios. Hydrological impacts of climate change differ through the wetland over short distances depending on the degree of groundwater/surface-water interaction. Discrete areas of groundwater upwelling are associated with an exaggerated response of water levels to climate change compared to non-upwelling areas. These are coincident with regions where a weathered chalk layer, which otherwise separates two main aquifers, is absent. Simulated water levels were linked to requirements of the MG8 plant community and Desmoulin’s whorl snail (Vertigo moulinsiana) for which the site is designated. Impacts on each are shown to differ spatially and in line with hydrological impacts. Differences in water level requirements for this vegetation community and single species highlight the need for separate management strategies in distinct areas of the wetland.

Hydrological Processes and Climate Change in Hydrographic Regions of Brazil

The objective of this work is to assess the impacts of IPCC AR5 climate change scenarios on water resources and hydrological processes across the entire Brazilian territory. Hydrological simulations are carried out in total drainage area of about 11,535,645 km2 and average stream flow of about 272,460 m3/s. The study area consists of different climates and land covers such as the Amazon Forest, Northeast Semiarid, Brazilian Savannah, Pantanal wetlands and temperate climate in the South. The atmospheric forcing to drive the large-scale hydrological model MGB-IPH is derived from the downscaling of two global climate models, HadGEM2-ES and MIROC5, by the Eta Regional Climate Model, at 20 km resolution. The Eta model provided the downscaling of the baseline (1961-1990) and three time-slices (2011-2040, 2041-2070 and 2071-2099). These projections adopted two emission scenarios, the RCP 4.5 and RCP 8.5. The change in the average and extremes of precipitation, evapotranspiration, rates of river discharge and soil moisture were assessed. The simulations showed the response of the hydrographic regions due to change of precipitation and potential evapotranspiration in the scenarios. Water availability decreases in almost the entire study area (exception for the South) and the major basins for hydroelectric power generation are affected. The Northwest, Amazon and a small area along the Northeast Atlantic coast exhibited intensification of the extremes discharges, where the anomaly is positive for high-flow (Q10) and negative for low-flow (Q95). The results highlight the most climatic sensitive regions in Brazil in terms of hydrological variables and water resources.

Driving factors for runoff decline in the Upper Hanjiang basin, a major water source for the South-to-North Water Diversion Project in China

Abstract. With dramatic changes in climate and land-cover patterns around the world, it is of great significance to evaluate the corresponding influence on runoff change as water resources have become a strategic resource. We analysed the runoff change driven by landscape change and climate variation in Hanjiang River basin, which is the water source area of the central route of the South-to-North Water Diversion Project in China. Results show that the runoff decreased greatly from 1960 to 2012 in all the six selected sub-catchments. Attribution analysis results show that reduction of precipitation contributed to the catchment runoff decrease by 39.5–64.9% and landscape change, represented by increase of the parameter in the mathematical Budyko function contributed to the runoff decrease by 34.4–63.3%, while potential evapotranspiration change had a slightly negative contribution. In addition, the contribution is spatially variable from downstream to upstream. We conclude with a qualitative description about how water availability changes under changing landscape and climate conditions, and focus on the impact of vegetation cover change.

Temporal Changes in Streamflow and Attribution of Changes to Climate and Landuse in Wisconsin Watersheds

AbstractPrevious historic trends analyses on 21st Century hydrologic data in the United States generally focus on annual flow statistics and have continued to use USGS hydro‐climatic data network (HCDN) stations, although post‐1988 diversions and runoff regulations are not reflected in the HCDN. Using a more recent dataset, Geospatial Attributes of Gages for Evaluating Streamflow, version II (GAGES II), compiled by Falcone (2012), which includes more watersheds with reference conditions, a comprehensive analysis of changes in seasonal, and annual streamflow in Wisconsin watersheds is demonstrated. Given the pronounced influence of seasonal hydrology in Wisconsin watersheds, the objective of this study is to elucidate the nature of temporal (annual, seasonal, and monthly) changes in runoff. Considerable temporal and regional variability was found in annual and seasonal streamflow changes between the two historic periods 1951‐1980 and 1981‐2010 considered in the study. For example, the northern watersheds show relatively small changes in streamflow discharge ranging from −6.0 to 4.2%, while the southern watersheds show relatively large increases in streamflow discharge ranging from 13.1 to 18.2%. To apportion streamflow changes to climate and nonclimatic factors, a method based on potential evapotranspiration changes is demonstrated. Results show that nonclimatic factors account for more than 60% of changes in annual runoff in Wisconsin watersheds considered in the study.

Understanding hydrological trends by combining the Budyko hypothesis and a stochastic soil moisture model

The actual evapotranspiration and runoff trends of five major basins in China from 1956 to 2000 are investigated by combining the Budyko hypothesis and a stochastic soil moisture model. Based on the equations of Choudhury and Porporato, the actual evapotranspiration trends and the runoff trends are attributed to changes in precipitation, potential evapotranspiration, rainfall depth and water storage capacity which depends on the soil water holding capacity and the root depth. It was found that the rainfall depth increased significantly in China during the past 50 years, especially in southern basins. Contributions from changes in the water storage capacity were significant in basins where land surface characteristics have changed substantially due to human activities. It was also observed that the actual evapotranspiration trends are more sensitive to precipitation trends in water-limited basins, but more sensitive to potential evapotranspiration trends in energy-limited basins.Editor D. Koutsoyiannis; Associate editor A. Porporato

THE EFFECT OF DEFICIT IRRIGATION ON THE FLOWERING BEHAVIOUR OF TWO DAY-NEUTRAL AND ONE EVERBEARING STRAWBERRY CULTIVAR

To determine the effect of deficit irrigation on the flowering behaviour of day neutral and everbearing strawberries, three cultivars (‘Albion’, ‘Evie 2’ and ‘Portola’) were exposed to one of three irrigation regimes; 100% (control), 80% (mild stress), 60% (severe stress) evapotranspiration (ETp) for a period of five weeks. The plants were then allowed to reach maximum water capacity before drought stress treatments were repeated for a further five-week period. Irrigation was adjusted daily in accordance with changes in potential evapotranspiration; calibration coefficients were determined weekly by changes in weight of the pots over a 24-hour period. The number of flowers and berries produced was recorded twice weekly and results are presented for flower numbers over a 12-week period. Application of either the mild or severe deficit treatments did not advance flowering in any of the cultivars tested. A mild stress had no effect on the flower numbers but the severe stress caused a reduction in number up to eight weeks after the deficit treatments. However following the recovery period there was no effect on flower quantity.

Climate change impacts on runoff in West Africa: a review

Abstract. This review summarizes the impacts of climate change on runoff in West Africa, assesses the uncertainty in the projections and describes future research needs for the region. To do so, we constitute a meta-database made of 19 studies and 301 future runoff change values. The future tendency in streamflow developments is overall very uncertain (median of the 301 points is 0% and mean +5.2%), except for (i) the Gambia River, which exhibits a significant negative change (median = −4.5%), and (ii) the Sassandra and the Niger rivers, where the change is positive (+14.4% and +6.1%). A correlation analysis revealed that runoff changes are tightly linked to changes in rainfall (R = 0.49), and to a smaller extent also to changes in potential evapotranspiration. Other parameters than climate – such as the carbon effect on plant water efficiency, land use dynamics or water withdrawals – could also significantly impact on runoff, but they generally do not offset the effects of climate change. In view of the potential changes, the large uncertainty therein and the high vulnerability of the region to such changes, there is an urgent need for integrated studies that quantify the potential effects of these processes on water resources in West Africa and for more accuracy in climate models rainfall projections. We especially underline the lack of information concerning projections of future floods and droughts, and of interannual fluctuations in streamflow.

Changing water availability during the African maize-growing season, 1979–2010

Understanding how global change is impacting African agriculture requires a full physical accounting of water supply and demand, but accurate, gridded data on key drivers (e.g., humidity) are generally unavailable. We used a new bias-corrected meteorological dataset to analyze changes in precipitation (supply), potential evapotranspiration (, demand), and water availability (expressed as the ratio ) in 20 countries (focusing on their maize-growing regions and seasons), between 1979 and 2010, and the factors driving changes in . Maize-growing areas in Southern Africa, particularly South Africa, benefitted from increased water availability due in large part to demand declines driven primarily by declining net radiation, increasing vapor pressure, and falling temperatures (with no effect from changing windspeed), with smaller increases in supply. Sahelian zone countries in West Africa, as well as Ethiopia in East Africa, had strong increases in availability driven primarily by rainfall rebounding from the long-term Sahelian droughts, with little change or small reductions in demand. However, intra-seasonal supply variability generally increased in West and East Africa. Across all three regions, declining net radiation contributed downwards pressure on demand, generally over-riding upwards pressure caused by increasing temperatures, the regional effects of which were largest in East Africa. A small number of countries, mostly in or near East Africa (Tanzania and Malawi) experienced declines in water availability primarily due to decreased rainfall, but exacerbated by increasing demand. Much of the reduced water availability in East Africa occurred during the more sensitive middle part of the maize-growing season, suggesting negative consequences for maize production.

Decoupling Streamflow Responses to Climate Variability and Land Use/Cover Changes in a Watershed in Northern China

AbstractRestored annual streamflow (Qr) and measured daily streamflow of the Chaohe watershed located in northern China and associated long‐term climate and land use/cover data were used to explore the effects of land use/cover change and climate variability on the streamflow during 1961‐2009. There were no significant changes in annual precipitation (P) and potential evapotranspiration, whereas Qr decreased significantly by 0.81 mm/yr (p < 0.001) over the study period with a change point in 1999. We used 1961‐1998 as the baseline period (BP) and 1999‐2009 the change period (CP). The mean Qr during the CP decreased by 39.4 mm compared with that in the BP. From 1979 to 2009, the grassland area declined by 69.6%, and the forest and shrublands increased by 105.4 and 73.1%, respectively. The land use/cover change and climate variability contributed for 58.4 and 41.6% reduction in mean annual Qr, respectively. Compared with the BP, median and high flows in the CP decreased by 38.8 and up to 75.5%, respectively. The study concludes that large‐scale ecological restoration and watershed management in northern China has greatly decreased water yield and reduced high flows due to the improved land cover by afforestation leading to higher water loss through evapotranspiration. At a large watershed scale, land use/cover change could play as much of an important role as climate variability on water resources.

Quantifying the relative impact of climate and human activities on streamflow

Summary The objective of this study is to quantify the role of climate and human impacts on streamflow conditions by using historical streamflow records, in conjunction with trend analysis and hydrologic modeling. Four U.S. states, including Indiana, New York, Arizona and Georgia area used to represent various level of human activity based on population change and diverse climate conditions. The Mann–Kendall trend analysis is first used to examine the magnitude changes in precipitation, streamflow and potential evapotranspiration for the four states. Four hydrologic modeling methods, including linear regression, hydrologic simulation, annual balance, and Budyko analysis are then used to quantify the amount of climate and human impacts on streamflow. All four methods show that the human impact is higher on streamflow at most gauging stations in all four states compared to climate impact. Among the four methods used, the linear regression approach produced the best hydrologic output in terms of higher Nash–Sutcliffe coefficient. The methodology used in this study is also able to correctly highlight the areas with higher human impact such as the modified channelized reaches in the northwestern part of Indiana. The results from this study show that population alone cannot capture all the changes caused by human activities in a region. However, this approach provides a starting point towards understanding the role of individual human activities on streamflow changes.

Future potential evapotranspiration changes and contribution analysis in Zhejiang Province, East China

AbstractPotential evapotranspiration is an important component of hydrological modeling. In this study, the objective is to project potential evapotranspiration in the future period 2011–2040 and understand their changes in Zhejiang Province, East China. The sensitivity of potential evapotranspiration to five climatic variables (solar radiation, daily minimum and maximum air temperature, relative humidity, and wind speed) is analyzed based on observation data from 1955–2008 using a global sensitivity analysis method, Sobol's method. The changes in potential evapotranspiration during the future period are generated using one regional climate model, Providing Regional Climates for Impacts Studies, with two global climate models, ECHAM5 and Hadley Centre Coupled Model version 3, and their causes are analyzed based on sensitivity analysis results. Global sensitivity analysis results reveal substantial spatial‐temporal variations in the sensitivity of potential evapotranspiration to climatic variables and unignorable interactions among climatic variables. Rather similar spatial change patterns of annual mean potential evapotranspiration (PET) are generated for both general circulation models; however, seasonal or monthly changes are very different due to different spatial‐temporal changes in climatic variables. Different contributory sources to potential evapotranspiration changes are identified at different months and stations; the PET changes in 2011–2040 are mainly due to three climatic variables including solar radiation, relative humidity, and daily minimum temperature.

Identification of Streamflow Response to Climate Change and Human Activities in the Wei River Basin, China

In this study, the calibration and validation period with stable underlying surface conditions was determined by using a statistically significant change point of the annual streamflow in several catchments of the Wei River basin (WRB). The effects of climate changes and human activities on streamflow were estimated by using the sensitivity-based method and the dynamic water balance model, respectively. The contributions of climate effects and human activities effects on streamflow were also investigated. The results showed that almost all the catchments exhibited significant decreasing trend of streamflow in the early 1990s. The streamflow was more sensitive to changes in precipitation than changes in potential evapotranspiration (PET). Effects of climate due to changes in precipitation and PET are weak in Linjiacun, Weijiabao and Xianyang catchments, while it is strong in the catchments controlled by other hydrological stations, accounting for more than 40 % of streamflow reduction. Effects of human activities on streamflow in Linjiacun, Weijiabao, Xianyang and Zhangjiashan catchments accounted for more than 50 % of the streamflow reduction. The study provides scientific foundation to understand the causes of water resources scarcity and useful information for the planning and management of water resources in the ecological fragile arid area.

Calculation of Forest Potential Evapotranspiration of Okinawa in Japan Using the Penman Equation

The northern area of Okinawa Island is a unique forest area in Japan, with a distinctive ecosystem and subtropical climate. The area is a central region of forestry in Okinawa Prefecture. However, quantitative evaluation of the effects of the forest environment is inadequate. The authors began meteorological observation of this forested area to address this situation by setting up a weather station in 2009. In this study, we performed research on one of the major factors of the water cycle in forest ecosystems, evapotranspiration. We calculate seasonal changes in potential evapotranspiration through analysis of data from our weather station in 2013, because all measurement elements were assembled. To calculate potential evapotranspiration, we used the Penman equation. We found that the potential evapotranspiration in this forest area was 1170.5 mm in 2013. The mean temperature in 2013 was 20.7°C, yearly average relative humidity was 84.7%, and average wind speed was 1.40 m/s. Regarding the amount of evapotranspiration in the forests of northern Okinawa Island, which has not been previously obtained, it has become possible to calculate the amount of potential evapotranspiration using the Penman equation.

Climate change and river flooding: part 1 classifying the sensitivity of British catchments

Effective national and regional policy guidance on climate change adaptation relies on robust scientific evidence. This two-part series of papers develops and implements a novel scenario-neutral framework enabling an assessment of the vulnerability of flood flows in British catchments to climatic change, to underpin the development of guidance for the flood management community. In this first part, the sensitivity of the 20-year return period flood peak (RP20) to changes in precipitation (P), temperature (T) and potential evapotranspiration (PE) is systematically assessed for 154 catchments. A sensitivity domain of 4,200 scenarios is applied combining 525 and 8 sets of P and T/PE mean monthly changes, respectively, with seasonality incorporated using a single-phase harmonic function. Using the change factor method, the percentage change in RP20 associated with each scenario of the sensitivity domain is calculated, giving flood response surfaces for each catchment. Using a clustering procedure on the response surfaces, the 154 catchments are divided into nine groups: flood sensitivity types. These sensitivity types show that some catchments are (very) sensitive to changes in P but others buffer the response, while the location of catchments of the same type does not show any strong geographical pattern. These results reflect the range of hydrological processes found in Britain, and demonstrate the potential importance of catchment properties (physical and climatic) in the propagation of change in climate to change in floods, and so in characterising the sensitivity types (covered in the companion paper).

Climate change and river flooding: Part 2 sensitivity characterisation for british catchments and example vulnerability assessments

This paper is the second of a series describing a scenario-neutral methodology to assess the sensitivity and vulnerability of British catchments to changes in flooding due to climate change. In paper one, nine flood sensitivity types were identified from response surfaces generated for 154 catchments. The response surfaces describe changes in 20-year return period flood peaks (RP20) in response to a large set of changes in precipitation, temperature and potential evapotranspiration. In this paper, a recursive partitioning algorithm is used to link families of sensitivity types to catchment properties, via a decision tree. The tree shows 85 % success characterising the four sensitivity families, using five properties and nine paths. Catchment annual average rainfall is the primary partitioning factor, with drier catchments having a more variable response to climate (precipitation) change than wetter catchments and higher catchment losses and permeability being aggravating factors. The full sensitivity-exposure-vulnerability methodology is illustrated for two catchments: sensitivity is estimated by using the decision tree to identify the sensitivity family (and its associated average response surface); exposure is defined from a set of climate model projections and combined with the response surface to estimate the resulting impacts (changes in RP20); vulnerability under a range of adaptive capacity thresholds is estimated from the set of impacts. Even though they are geographically close, the two catchments show differing vulnerability to climate change, due to their differing properties. This demonstrates that generalised response surfaces characterised by catchment properties are useful screening tools to quantify the vulnerability of catchments to climate change without the need to undertake a full climate change impact study.

Promjene i trendovi klimatskih elemenata i indeksa na podrucju sredozemne Hrvatske

Climate is complex ecological factor described by different climate elements and phenomena that effect development of vegetation and its natural distribution. Climate elements the most important for vegetation are air temperatures, amount of precipitations, air humidity and wind. The aim of this research was to determine trends and changes of climate elements in the region of Mediterranean Croatia. Meteorological stations with the longest monitoring period in the region of the Mediterranean Croatia were chosen in order to determine trends of the climate elements. Decreasing or increasing trends of individual climate elements and indices were analysed using linear trend of regression analysis. Climate elements and indices of the referent line were compared with period between 1991 and 2010. At all meteorological stations was found negative trend of annual amount of precipitations and Lang ́s rain factor. Exception was meteorological station Rijeka with increase of annual amount of precipitations and Lang ́s rain factor. Trends of air temperatures at all meteorological stations were positive. Values of Lang ́s rain factor in the researched region are decreasing resulting higher aridity of the region. Trends of potential evapotranspiration are significant and positive at all meteorological stations. Changes of air temperatures are more noticeable than those of precipitations, while changes of potential evapotranspiration are more noticeable than those of Lang ́s rain factor. Forest vegetation and crop plants of Mediterranean Croatia are adapted to certain climate conditions predominating in this region. This conditions change through time effecting growth and development of all organisms.