Changes In Reference Evapotranspiration Research Articles

Reference evapotranspiration change and the causes across the Yellow River Basin during 1957–2008 and their spatial and seasonal differences

As an indicator of atmospheric evaporating capability over a hypothetical reference surface, reference evapotranspiration (ET0) is the most important hydrological and meteorological variable to reflect climate change. This is particularly true for the Yellow River Basin, which faces serious water shortages and is vulnerable to climate change. In this study, the ET0 at 80 sites during 1957–2008 in the Yellow River Basin was calculated using the Penman‐Monteith method with the calibrated Angstrom coefficients. Spatial and seasonal patterns of changes in ET0 as well as the concerned climatic variables are specially focused on using advanced statistical tests and GIS method. The entire Yellow River Basin is characterized by complicated spatial variability in the change of ET0. Significant negative trends are mainly distributed in the southeast corner, northern side, and midwest of the Yellow River Basin, while significant increases of ET0 mainly occur in the middle part and southwest corner of the basin. Still, no coherent spatial patterns in ET0 trends are seen in any season. The dominance of warming trends in temperature and decreasing trends in wind speed and sunshine duration can be found in the basin. Relative humidity presents insignificant or weak trends at many sites but with a mixed spatial structure of positive and negative trends at both annual and seasonal scales. The combined effects of climatic variables to ET0 changes and their spatial and seasonal variability are revealed by further analysis of sensitivity of ET0 to climatic variables and the contribution of climatic variables to ET0 changes over six homogenous regions identified by a rotated empirical orthogonal function (REOF) clustering method on annual and seasonal scales. The decline of surface wind speed offsets the increasing effect of the temperature increase and is mainly responsible for the ET0 reduction in the west and north of the Loess Plateau. The reduced sunshine duration is the leading factor for ET0 decrease in the middle‐lower Yellow River Plain, especially during the summer time. The increasing mean temperature plays the most important role in the ET0 increase in the source area of the Yellow River Basin. Furthermore, regional actual evapotranspiration and ET0 present complementary behavior, but does not accurately fall in the 1:1 complementary relationship of the Bouchet's hypothesis, especially for the high elevation subregions. In addition, although precipitation changes are the main driving factors for drought variation, increasing ET0 intensified the drought in middle regions.

Future climate change in the Mediterranean area: implications for water use and weed management

Results obtained within research activity from the Climesco Italian Project are summarized. These results suggest that in regards to the impact of climate change in the Mediterranean area, a decrease of water availability and a more frequent occurrence of drought periods are expected. In order to describe the main effects of climate change on water use in some agro-ecosystems in the Mediterranean area we showed that the Penman-Monteith equation can be modified to simulate future changes in reference evapotranspiration by recalibration of the crop resistive parameter. Moreover, the use of adjusted crop coefficients (Kc) can help quantify the climate change impact on water use for irrigated crops grown in Southern Italy and elsewhere in the Mediterannean. For this region temperature rise and the concomitant expected rainfall reduction may lead to an increase yearly potential water deficits. For autumn-spring crops a further increase of water deficit is not expected. In contrast for a significant increase of water deficit, and thus of irrigation needs, is expected for spring-summer crops. Another aspect considered in this review is how in the Mediterranean area, drought conditions and warmer temperatures will alter the competitive balance between crops and some weed species. We report experimental data showing how weed aggressiveness and competition is already increasing due to warmer temperatures in the Mediterranean region.

Reference Evapotranspiration Changes in the Haihe River Basin during Past 50 Years

In this paper, temporal trend of annual reference evapotranspiration (ET0) calculated using the FAO Penman-Monteith equation with the observed daily meteorological data at six stations (i.e. Datong, Weichang, Qinhuangdao, Tianjin, Beijing and Huimin) of Haihe River Basin, China, were detected with the help of parametric t-test and Mann-Kendall (MK) analysis. The six stations were divided in three different classes representing mountain (Datong and Weichang), continental (Beijing and Huimin) and coastal areas (Qinhuangdao and Tianjin), respectively. The result shows that there was a significant upward trend in ET0 at mountain area of the Haihe River Basin. On the contrary, a significant downward trend in ET0 can be found at coastal area. Moreover, the analyses of ET0 at continental area indicate that after 1960, ET0 of Beijing showed a sharp significant increase, while a moderate variation was presented for ET0 at Huimin.

Causes of decreased reference evapotranspiration and pan evaporation in the Jinghe River catchment, northern China

This study focused on the changes of reference evapotranspiration (ET0) and pan evaporation (ETpan) to study the impacts of climate change on the hydrological cycle in the Jinghe River catchment. Based on the Penman–Monteith equation, the ET0 was calculated. The temporal trend and spatial distribution of ET0 and Epan measured with a 20-cm pan were examined at the 14 stations during 1957–2005. The effects of meteorological factors on the variation of ET0 were determined by analyzing the trends in themselves with comparison between original climate and detrended climate scenarios and then their sensitivity to ET0. Both the ET0 and Epan showed remarkable decreasing trends from 1957 to 2005 and their decreasing rate was 40.9 and 17.7 mm per 10 years, respectively. Trend analysis of meteorological factors exhibited that the reduction in ET0 and ETpan was principally caused by both significant decreases in wind speed and sunshine hours. Furthermore, the decreasing trend of ET0 was mainly dominated by the significant decrease in wind speed with high sensitivity, to a less extent, by the decrease in net radiation. Although relative humidity is one of the most sensitive variables, its effect on ET0 was negligible because of its temporal constancy. The contribution of wind speed reduction to decreased ET0 has increased from 50 to 76.1%, but net radiation, by contrast, decreased from 50 to 23.9%.

Effects of rising atmospheric CO2 on crop evapotranspiration in a Mediterranean area

Abstract In the assessment of plant response to the climate changes, the effects of CO 2 increase in the atmosphere and the subsequent rise of temperatures must be taken into account for their effects on crop physiology. In Mediterranean areas, a decrease of water availability and a more frequent occurrence of drought periods are expected. The objective of this study was to assess the impact of elevated CO 2 concentration and high temperature on reference evapotranspiration (ETo) and crop evapotranspiration (ETc) in the Mediterranean areas. The Penman–Monteith equation was used to simulate the future changes of reference evapotranspiration (ETo) by the recalibration of the canopy resistance parameter. Besides, crop coefficients (Kc) were adjusted according to the future climate trend. Then the modified empirical model (ETc = ETo × Kc) was applied providing an effective quantification of the climate change impact on water use of irrigated crops grown in Mediterranean areas. In the studied area, water use assessment was carried out for the period from 1961 to 2006 (measured data) and for a period from 2071 until 2100 (simulated data), showing a future climatic scenario. Water and irrigation use of crops will change as a function of climate changes, thermal needs of single crops and time of the year when they grow. Climate simulation model foresees the tendency for a significant increase of temperatures and a decrease of total year rainfall with a change of their distribution. The temperature increase and the concomitant expected rainfall decrease lead to a rise of year potential water deficit. About the autumn–spring crops, as wheat, a further increase of water deficit, is not expected. On the contrary, for spring–summer crops as tomato, a significant increase of water deficit and thus of irrigation need, is foreseen. Actually, for crops growing in that period of the year, the substantial rise of evapotranspiration demand cannot be compensated by crop cycle reduction and partial stomatal closure.

Relationships Between Climatic Variables and Sap Flow, Stem Water Potential and Maximum Daily Trunk Shrinkage in Lemon Trees

The feasibility of obtaining sap flow (SF), maximum daily trunk shrinkage (MDS) and midday stem water potential (Ψstem) baselines or reference values for use in irrigation scheduling was studied in adult Fino lemon trees (Citrus limon (L.) Burm. fil.) grafted on sour orange (C. aurantium L.) rootstocks. Plants were irrigated daily above their water requirements in order to obtain non-limiting soil water conditions. The results indicated that baselines for plant-based water status indicators (MDS, SF and Ψstem) can be obtained, even though there was a certain scattering of the data points representing the relations between the plant-based measurements and the environmental variables (reference evapotranspiration, solar radiation, vapour pressure deficit and temperature). SF was more closely associated with changes in the studied evaporative demand variables than were MDS and Ψstem. SF and Ψstem were more closely correlated with changes in reference evapotranspiration (ETo) (r2 = 0.93 and 0.79, respectively), while MDS behaviour was best correlated with mean daily air temperature (Tm) (r2 = 0.76). Increases in the evaporative demand induced more negative Ψstem values and, as a consequence, SF increased, which, in turn, was translated into an increase in MDS. This confirmed that SF and MDS were very good predictors of the plant water status during the observation period and their continuous recording offers the promising possibility of their use in automatic irrigation scheduling in lemon trees.