Variability Of Evapotranspiration Research Articles

Evapotranspiration of Winter Wheat in the Semi-Arid Southeastern Loess Plateau Based on Multi-Source Satellite Data

Continuous monitoring of evapotranspiration (ET) at high spatio-temporal resolutions is vital for managing agricultural water resources in arid and semi-arid regions. This study used the enhanced spatial and temporal adaptive reflectance fusion model (ESTARFM) to calculate the ET of winter wheat between the green-up and milk stages in Linfen Basin, a typical, semi-arid area of the Loess Plateau, at temporal and spatial resolutions of 30 m and 8 d, respectively. We then analyzed the impact of meteorological factors on ET and its variation during the main growth period of winter wheat. The fused ET data displayed the spatial details of the OLI ET data better and could accurately reflect ET variation and local sudden variations during the main growth period of winter wheat. Moreover, winter wheat ET in rain-fed areas is more heavily influenced by meteorological factors, and the effect is more direct. Affected by the synergistic effect of wind velocity, precipitation, and temperature, the ET of winter wheat in rain-fed area was lower in the green-up stage. Then, ET gradually increased, reaching its maximum in the heading–grain filling stage. At the jointing stage, temperature had a significant effect on ET. A combination of precipitation and temperature had the greatest impact on the ET of winter wheat in the heading–filling stage. In the milk stage, meteorological factors had a minor impact on ET. This study serves as a reference for ET in winter wheat in semi-arid areas and its influencing meteorological factors, which can assist in drought mitigation and regional food security strategies.

Characterization of Evapotranspiration in the Orange River Basin of South Africa-Lesotho with Climate and MODIS Data

Evapotranspiration (ET) is crucial to the management of water supplies and the functioning of numerous terrestrial ecosystems. To understand and propose planning strategies for water-resource and crop management, it is critical to examine the geo-temporal patterns of ET in drought-prone areas such as the Upper Orange River Basin (UORB) in South Africa. While information on ET changes is computed from directly observed parameters, capturing it through remote sensing is inexpensive, consistent, and feasible at different space–time scales. Here, we employed the Moderate Resolution Imaging Spectroradiometer (MODIS)-derived spectral indices within Google Earth Engine (GEE) to analyze and characterize patterns of ET over the UORB from 2003 to 2021, in association with various climatic parameters. Our results show spatially consistent ET patterns with the Vegetation Condition Index (VCI), with lower values in the west, increasing toward the eastern section of the basin, over the Lesotho highlands. We noted that the UORB faced significant variability in ET and VCI during pronounced drought episodes. The random forests (RF) model identified precipitation, temperature, Standardized Precipitation Index (SPI)-6, Palmer Drought Severity Index (PDSI), and VCI as variables of high importance for ET variability, while the wavelet analysis confirmed the coherence connectivity between these variables with periodicities ranging from eight to 32 months, suggesting a strong causal influence on ET, except for PDSI, that showed an erratic relationship. Based on the sequential Mann–Kendall test, we concluded that evapotranspiration has exhibited a statistically downward trend since 2011, which was particularly pronounced during the dry periods in 2015–2016, 2019, and 2021. Our study also confirmed the high capacity of the GEE and MODIS-derived indices in mapping consistent geo-temporal ET patterns.

Investigation into climate change effects on carbon and water fluxes, and water use efficiency of the temperate grassland ecosystems in Inner Mongolia of China

Abstract The daily gross primary productivity (GPP) and evapotranspiration (ET) in the Xilingol grassland ecosystem of Inner Mongolia were simulated using the BioGeochemical Cycles (Biome-BGC) model for 2003–2019 and under future climate-change scenarios. The system was optimized using the System Response Parameter Calibration Method (SRPCM). The temporal variations of GPP, ET and water use efficiency (WUE) were investigated, and the impacts of precipitation and temperature were explored. Results showed that (i) the BIOME-BGC model performed better when optimized using the SRPCM than by applying the Model-Independent Parameter Estimation approach (PEST); (ii) GPP and ET at annual and seasonal scales showed an insignificant increasing trend; (iii) WUE at the annual scale and in growing seasons showed an insignificant increasing trend and a slight decreasing trend in non-growing seasons; (iv) annual GPP and ET were more sensitive to changes in precipitation than changes in temperature with WUE keeping relatively stable with years; (v) precipitation is a critically controlling factor for GPP and ET in growing seasons and for ET and WUE in non-growing seasons; and (vi) combined temperature and precipitation changes had greater impacts on GPP/ET/WUE than individual changes.

Effects of landscape attributes and climate variables on catchment hydrology

Catchments characteristics, such as geomorphology, geology, soil, land use, and climatic variables, play an important role in total stream flow responses, a critical resource for people and the environment. Most of the previous literatures were applied a conventional statistical regression model to assess the relationship between landscape-climate descriptors, and streamflow and PET. However, a conventional statistical regression model didn’t consider dependence of explanatory variables that were collected or extracted across both space and time. This paper investigated the impacts of landscape attributes and climate variables on catchment scale temporal variation of total streamflow and spatio-temporal variation of potential evapotranspiration (PET) in the Mille catchment using multiple linear regression techniques, and the importance of this study was to test spatial autocorrelation in the spatial regression model which is required to properly assess and quantify the relationship between hydrological regime response components and Landscape-climate descriptors in a catchment with topographically complex, and high spatio-temporal climatic variation like in our case study area, the Mille catchment. Statistical regression analysis revealed significant relationships between streamflow and climate variables, especially with rainfall. Mean maximum temperature is the most dominant factor controlling temporal variation of potential evapotranspiration at a monthly scale, whereas NDVI is the most significant factor that controls the spatial variability of PET. The multiple regression model shows that 91.1% of temporal variation in streamflow was accounted for rainfall, whereas, 96.6% and 78.4% of temporal and spatial variation in potential evapotranspiration was accounted for in maximum temperature and NDVI, respectively. Methods also can be applied to catchments with similar landscape attributes and climate variables.

Varying performance of eight evapotranspiration products with aridity and vegetation greenness across the globe

The wide application of the evapotranspiration (ET) products has deepened our understanding of the water, energy and carbon cycles, driving increased interest in regional and global assessments of their performance. However, evaluating ET products at a global scale with varying levels of dryness and vegetation greenness poses challenges due to a relative lack of reference data and potential water imbalance. Here, we evaluated the performance of eight state-of-the-art ET products derived from remote sensing, Land Surface Models, and machine learning methods. Specifically, we assessed their ability to capture ET magnitude, variability, and trend, using 1,381 global watershed water balance ET as a baseline. Furthermore, we created aridity and vegetation categories to investigate performance differences among products under varying environmental conditions. Our results demonstrate that the spatial and temporal performances of the ET products were strongly affected by aridity and vegetation greenness. The poorer performances, such as underestimation of interannual variability and misjudged trend, tend to occur in abundant humidity and vegetation. Our findings emphasize the significance of considering aridity and vegetation greenness into ET product generation, especially in the context of ongoing global warming and greening. Which hopefully will contribute to the directional optimizations and effective applications of ET simulations.

Lumped variable representing the integrative effects of climate and underlying surface system: Interpreting Budyko model parameter from earth system science perspective

Parametric Budyko models express the ratio of annual actual evapotranspiration (ET) to annual precipitation (P) as a function of dryness index, i.e., annual potential evapotranspiration (ET0)/P (denoted as DI) and model parameter at a catchment. Generally the model parameter is assumed to reflect factors besides DI, especially the underlying surface features. However, recent studies have revealed its relations with DI, which forces us to further examine its connotation. We revisited the derivation of three parametric Budyko equations and found that model parameter was essentially a lumped variable representing the integrative effects of climate and underlying surface system. Then taking 15 typical catchments in the Chinese Loess Plateau as the study area, we explored the controls of Budyko-Fu model parameter (w) and their differences in temporal (between-years) and spatial (between-catchments) processes. As expected, w was affected by both climate and underlying surface factors. Specifically, DI, climate seasonality and asynchrony index (SAI) and Normalized Difference Vegetation Index (NDVI) influenced w in both temporal and spatial processes; besides, catchment relief ratio (CR) and relative infiltration capacity (ks/ir) also affected w in spatial process. SAI was strongly correlated with DI and its close relationship with w essentially conveyed DI's influence. Considering the inherent connection between DI and the inverse w, DI was not included in the empirical formula for w. SAI and NDVI remained independent in temporal process and the empirical formula for w with these two factors modeled annual ET well (MAE = 15.03 mm). In spatial process, SAI, NDVI, CR, and ks/ir were interconnected and by using any one the spatial variation of ET was captured adequately (MAE = 11.16, 11.74, 11.27, and 10.43 mm, respectively). Our results demonstrate theoretically and empirically the importance of viewing Budyko model parameter from earth system science perspective and focusing on the differences of its controls in spatiotemporal processes.

Compensating Effects Between Climate and Underlying Characteristics on Watershed Water Loss

AbstractIdentifying climate impacts on watershed water loss (evapotranspiration) generally involves the interactions with land surface change (e.g., vegetation dynamics and anthropogenic disturbances). Here, we seek to understand the compensating effects of climate and underlying characteristics on watershed evapotranspiration. In this regard, an analytical solution was derived by using a simple water‐energy partitioning framework for separating the individual contributions of climate and underlying characteristics to changes in watershed actual evapotranspiration between 1930–1970 and 1971–2008 over global 87 basins. We found that for the basins with larger increases in the total evapotranspiration, a positive underlying characteristic‐related contribution is dominant, rather than the climate‐related contribution. Tradeoffs (i.e., the compensating effects) between the contributions of climate and underlying characteristics to evaporative water loss were also identified in a few basins where there were relatively large changes in the underlying characteristic‐related evapotranspiration but only a slight change in the total evapotranspiration. This suggests that the climate contribution in evapotranspiration may offset the effect of underlying characteristic‐related changes over these basins, associated with stronger hydrological resilience under the double impacts of both climate and underlying factors. The results also indicate that the underlying characteristics exert greater contributions to the changes in the total evapotranspiration rather than the climate variation, and the underlying characteristic‐related evapotranspiration variations are closely related to the changes in reservoirs and land‐use types of the basins. The presented findings may provide an insightful understanding of interactions among climate, water, and underlying characteristics over global basins.

Correction: Li et al. How Has the Recent Climate Change Affected the Spatiotemporal Variation of Reference Evapotranspiration in a Climate Transitional Zone of Eastern China? ISPRS Int. J. Geo-Inf. 2022, 11, 300

In the published publication [...]

Quantifying Spatiotemporal Variation of Reference Evapotranspiration and Its Contributing Climatic Factors in Bangladesh during 1981–2018

Quantifying Spatiotemporal Variation of Reference Evapotranspiration and Its Contributing Climatic Factors in Bangladesh during 1981–2018

Calibration‐Free Priestley‐Taylor Method for Reference Evapotranspiration Estimation

AbstractThe Priestley‐Taylor (PT) method is widely used in hydrologic and crop models to calculate the atmospheric demand for water or reference evapotranspiration (ETo). However, the default value of the PT coefficient cannot assure reliability under different climatic regions due to the inadequate consideration of the aerodynamic components. In addition, the spatial and temporal evolution of PTa at global scales and how meteorological parameters affect its change have not been elucidated to date. Here, we derived an analytical expression of PTa following the Penman‐Monteith method, i.e., , where SA and represent the available energy and wind speed impacts on ETo, respectively. The daily Princeton Global Forcing data set (0.25° × 0.25°, 1948–2016) was used to validate PTa. The calibration‐free PTa improved the simulation of aerodynamic components and, consequently, the accuracy of ETo estimation. The gridded global monthly PTa data set is fully open‐source and can be incorporated into hydrologic and crop models. We also demonstrated that PTa under equilibrium or quasi‐equilibrium evaporation conditions (1.26 ± 0.04) was very close to the default value. The simplified PTa, with relative humidity as the only input, still performed better than the original method. Results indicated that the radiative component was the driving factor of the global ETo changes from 1948 to 2016. The impacts of available energy and wind speed on global ETo variations tend to intensify in a warming climate. Our findings have important implications for understanding the roles of radiative and aerodynamic components in the global ETo changes.

Meteorological Influences on Short-Term Carbon-Water Relationships in Two Forests in Subtropical China

Carbon and water fluxes in ecosystems are tightly coupled by gas diffusion through stomata. However, carbon–water (C–W) relationships vary largely across time scales, vegetation types, and regions. Subtropical forests in China play an important role in the global carbon and water cycles, yet studies of C–W relationships in this region remain limited. Here, we investigated summer-time C–W relationships in this region at two subtropical sites: the evergreen broadleaved forest at Dinghushan (23.17° N, 112.53° E, 300 m) and the evergreen coniferous forest at Qianyanzhou (26.74° N, 115.06° W, 106 m), using the flux tower data from the FLUXNET2015. The C–W relationship was examined using two measures. The first was daily water use efficiency (WUE), which is the ratio of daily gross primary productivity (GPP) to evapotranspiration (ET). The second was the correlation coefficient (r) of hourly GPP and ET. Our analysis showed that the daily WUE in the two forests ranged over 4–14 mg CO2 per g H2O, higher in the coniferous forest than in the broadleaved forest. The mean values of r for hourly C–W coupling were similar at the two forests, being 0.5–0.6, which suggests asynchronous diurnal variations in GPP and ET. Both daily WUE and r were modulated by meteorological conditions. In general, high radiation, air temperature, and humidity can reduce WUE at both sites. For the broadleaved forest, the most influential factor on WUE was VPD, followed by radiation, while in the coniferous forest, VPD, air temperature, and radiation were almost equally important. For hourly C–W coupling, VPD plays a significant role. The drier the air is, the weaker the coupling in the two forests. The daily WUE and hourly C–W coupling reflect the C–W relationship from different perspectives. Both showed the strongest response to VPD but with different sensitivity.

Evapotranspiration responses to CO2 and its driving mechanisms in four ecosystems based on CMIP6 simulations: Forest, shrub, farm and grass

Evapotranspiration (ET) is an essential process of the water cycle through which water is transferred from terrestrial ecosystems to atmosphere. However, in the climate context of increasing CO2 concentration (also called as a CO2-enriched climate), the variation of ET and its main drivers among different ecosystems remain unclear. This study analyzed the output data of the CMCC ESM2 model with a ridge regression method, and proposed the trends and drivers of ET in different ecosystems in a CO2-enriched climate. In particular, the temporal - spatial characteristics of ET and its primary drivers for different periods and wetness levels were revealed. With the rising of CO2 concentration, the atmospheric evapotranspiration demand increases, and the vegetation grows more luxuriantly. ET shows an overall upward trend, especially in the shrub ecosystems (7.41 mm decade−1). Our results show that the thermal conditions are the main driving factors for humid forest and shrub ecosystems whereas relative humidity (RH) is the main driving factor for arid farm and grass ecosystems. In terms of the average contribution in all periods, surface solar radiation contributes 26% and 41% to ET variation in forest and shrub ecosystems, and RH contributes 49% and 32% to ET variation in farm and grass ecosystems, respectively. Notably, with the increase of wetness levels, the contribution of water conditions on ET becomes smaller, while that of thermal conditions becomes larger. Correlation analysis shows that LAI impacts on ET are regulated by environmental factors, which reflects the complexity of ET change mechanism. Overall, these findings further provide a reference for rational planning of ecosystems and efficient utilization of water resources.

City-wide, high-resolution mapping of evapotranspiration to guide climate-resilient planning

The impacts of global change, including extreme heat and water scarcity, are threatening an ever-growing urban world population. Evapotranspiration (ET) mitigates the urban heat island, reducing the effect of heat waves. It can also be used as a proxy for vegetation water use, making it a crucial tool to plan resilient green cities. To optimize the trade-off between urban greening and water security, reliable and up-to-date maps of ET for cities are urgently needed. Despite its importance, few studies have mapped urban ET accurately for an entire city in high spatial and temporal resolution. We mapped the ET of Berlin, Germany in high spatial (10-m) and temporal (hourly) resolution for the year of 2019. A novel machine learning (ML) approach combining Sentinel-2 time series, open geodata, and flux footprint modeling was applied. Two eddy flux towers with contrasting surrounding land cover provided the training and testing data. Flux footprint modeling allowed us to incorporate comprehensive land cover types in training the ML models. Open remote sensing and geodata used as model inputs included Normalized Difference Vegetation Index (NDVI) from Sentinel-2, building height, impervious surface fraction, vegetation fraction, and vegetation height. NDVI was used to indicate vegetation phenology and health, as plant transpiration contributes to the majority of terrestrial ET. Hourly reference ET (RET) was calculated and used as input to capture the temporal dynamics of the meteorological conditions. Predictions were carried out using Random Forest (RF) regression. Weighted averages extracted from hourly ET maps using flux footprints were compared to measured ET from the two flux towers. Validation showed that the approach is reliable for mapping urban ET, with a mean R2 of 0.76 and 0.56 and a mean RMSE of 0.0289 mm and 0.0171 mm at the more vegetated site and the city-center site, respectively. Lastly, the variation of ET between Local Climate Zones (LCZs) was analyzed to support urban planning. This study demonstrated the capacity to map urban ET at an unprecedented high spatial and temporal resolution with a novel methodology, which can be used to support the sustainable management of green infrastructure and water resources in an urbanizing world facing climate change.

Climate, desertification, and local human development: evidence from 1564 regions around the world

How global warming affects human development is a central question for economists as well as social scientists. While most of the literature has focused on the impact of weather on income, less is known on the relationship between climate and local human development. This paper considers shocks in precipitation, temperature, and an original measure of soil aridity to first exploit the association between climate warming and human development, and second, on its dimensions. We show that while precipitations do not have a significant long-term impact on human development growth, variations in temperature and potential evapotranspiration negatively affect two of the three determinants of the Human Development Index, namely life expectancy at birth and education. These results suggest that other climate indicators, such as the potential evapo-transpiration of the soil, should be considered in addition to the standard indicators, when evaluating the localized economic effects of climate change.

Attributing Evapotranspiration Changes with an Extended Budyko Framework Considering Glacier Changes in a Cryospheric-Dominated Watershed

The retreat of glaciers has altered hydrological processes in cryospheric regions and affects water resources at the basin scale. It is necessary to elucidate the contributions of environmental changes to evapotranspiration (ET) variation in cryospheric-dominated regions. Considering the upper reach of the Shule River Basin as a typical cryospheric-dominated watershed, an extended Budyko framework addressing glacier change was constructed and applied to investigate the sensitivity and contribution of changes in environmental variables to ET variation. The annual ET showed a significant upward trend of 1.158 mm yr−1 during 1982–2015 in the study area. ET was found to be the most sensitive to precipitation (P), followed by the controlling parameter (w), which reflects the integrated effects of landscape alterations, potential evapotranspiration (ET0), and glacier change (∆W). The increase in P was the dominant factor influencing the increase in ET, with a contribution of 112.64%, while the decrease in w largely offset its effect. The contributions of P and ET0 to ET change decreased, whereas that of w increased when considering glaciers using the extended Budyko framework. The change in glaciers played a clear role in ET change and hydrological processes, which cannot be ignored in cryospheric watersheds. These findings are helpful for better understanding changes in water resources in cryospheric regions.

Phenology-Based Remote Sensing Assessment of Crop Water Productivity

The assessment of crop water productivity (CWP) is of practical significance for improving regional agricultural water use efficiency and water conservation levels. The remote sensing method is a common method for estimating large scale CWP, and the assessment errors in CWP by remote sensing originate mainly from remote sensing inversion errors in crop yield and evapotranspiration (ET). The phenological period is the important factor in crop ET and yield estimation. The crop coefficient (Kc) and harvest index (HI), which are closely related to different phenological periods, are considered during the processes of crop ET and yield estimation. The crop phenological period is detected from enhanced vegetation index (EVI) curves using Moderate Resolution Imaging Spectroradiometer (MODIS) data and Sentinel-2 data. The crop ET is estimated using the surface–energy balance algorithm for land (SEBAL) model and Penman‒Monteith (P-M) equation, and the crop yield is estimated using the dry matter mass–harvest index method. The CWP is calculated as the ratio of the crop yield to ET during the growing season. The results show that the daily ET and crop yield estimated from remote sensing images are consistent with the measured values. It is found from the variation in daily ET that the peaks appear at the heading period of wheat and maize, which are in good agreement with the rainfall and growth characteristics of the crop. The relationship between crop yield and ET shows a negative parabolic correlation, and that between CWP and crop yield shows a linear correlation. The average CWPs of wheat and maize are 1.60 kg/m3 and 1.39 kg/m3, respectively. The results indicate that the phenology-based remote sensing inversion method has a good effect on the assessment of CWP in Lixin County.

Assessing the variations of evapotranspiration and its environmental controls over a subalpine wetland valley in China

Dynamics in long-term evapotranspiration (ET) and its controlling variables are essential for understanding how a high-altitude wetlands ecosystem responds to climate change. The rising temperature is expected to agitate the regional hydrological cycle and water balance, particularly in the subalpine wetland valley of Jiuzhaigou, located in the transition zone between the northeast Qinghai-Tibet Plateau and the Sichuan Basin, Southwest China. Here, we used growing season multi-year (2013–2021) Bowen ratio data to assess the variability in ET and its key controlling parameters at different timescales in Jiuzhaigou valley. This study also explored the ratio of ET to precipitation (P). The wetland daily mean ET varied from 0.06 to 6.77 mm d−1, with a mean value of 2.64 mm d−1 for the nine years. Fluctuations in daily ET were primarily driven by available energy (net radiation, Rn), explaining 86 % of the variation. Seasonal patterns in ET were largely similar to environmental parameters, i.e., Rn, air temperature (Ta), and vapor pressure deficit (VPD), peaking in August with an interannual monthly mean value of 3.48 mm d−1. Interannual monthly mean ET had a strong positive linear relationship with Rn, Ta, and VPD, while there was no significant correlation with P on a growing season basis. Furthermore, monthly ET was shown to be regulated by Ta largely in high-temperature months and minimally in low-temperature months. The growing season ET varied interannually, and the ET to P ratio (i.e., ET/P) ranged between 0.52 and 1.16. Interannual variation in annual ET was controlled by Ta and P, which individually explained 73 and 61 % of the variation, respectively. The multiple regression model indicated that Ta and P together elucidated 92 % of the variation in annual ET. The increased sensitivity (e.g., regression slopes) of ET to P over 2014–2021 indicates that ET consumed most of P, which leads to decreasing runoff and streams drying up. This study clarifies the temporal dynamics in ET for wetlands and its environmental controls at multiple timescales. It is demonstrated that the proportion of ET could increase in response to increasing temperature without an associated increase in P, affecting local water balance. These results could potentially contribute to sustainable water management in high-altitude wetlands and environmental planning under future climate change.

Stem growth of Amazonian species is driven by intra-annual variability in rainfall, vapor pressure and evapotranspiration

Intra-annual distribution of precipitation in central Amazonia often leads to a short mild dry season and an increase in irradiance, temperature, and vapor pressure deficit; however, the accurate effect of intra-annual microclimatic variability on stem growth is still under investigation. The objective of this study was to determine how stem growth responds to monthly variations in microclimatic factors in the central Amazon. During five years (2008-2012) we measured diameter stem growth of 109 trees (26 species) and used principal component regression to evaluate the effect of microclimatic variability on stem growth in diameter. We found that the mean stem growth in diameter across species increased in response to an increase in rainfall and reference evapotranspiration, but it decreased with a rise in mean and minimum vapor pressure deficit. A contribution of this study is to show that even when irradiance and temperature had no significant effect on stem growth, small changes in vapor pressure deficit significantly affect stem growth. If the dry season becomes longer, as predicted by models, trees currently more sensitive to microclimatic variability associated with droughts would be the most affected by climate changes.

Controlling Factors of the Spatial‐Temporal Fluctuations in Evapotranspiration Along an Elevation Gradient Across Humid Montane Ecosystems

AbstractDiverse meteorological and vegetation conditions at different elevations and years lead to varied contributions of evapotranspiration (ET) to water yield in montane areas. However, how physical and biological factors affect interannual and elevational ET fluctuations in energy‐limited montane ecosystems remains unclear. Therefore, we quantified the spatial‐temporal patterns of ET and separated physical and biological controls on ET fluctuations using 5‐year (2015–2019) eddy‐covariance data and perturbation analysis on Mount Gongga, Southwest China. Coefficients of variations of annual ET were 0.85%–4.33%, suggesting the remarkable interannual consistencies of ET in all studied zones. In forest zones, decreased precipitation (P) contributed more than 85% of ET reduction in the severe dry year, while the decreased vapor pressure deficit and available energy accounted for 88%, 78%, and 87% of transpiration reductions in broadleaf forest, broadleaf and coniferous forest and coniferous forest, respectively, in the wet year. In shrubland, the positive influence from biological factors (41.37 mm) surpassed the negative impact from physical conditions (−16.46 mm), resulting in an increase in ET in the wet year. Mean annual ET was highest at 2,200 m a.s.l., 15.04% higher than that at 3,996 m a.s.l. From perturbation analysis, elevational ET fluctuations were smaller in low‐elevation (12.31 mm) and middle‐elevation (−31.18 mm) zones than that in high‐elevation zone (−99.86 mm). Large elevational ET reduction in high‐elevation zone mainly resulted from decreased P and leaf area index. In humid montane areas, although ET is predicted to increase, how interannual and elevational ET respond to climate changes should be considered separately.

Palmer-type soil modelling for evapotranspiration in different climatic regions of Kenya

Reference evapotranspiration (ET0) and real evapotranspiration (ET) are vital components in hydrological processes and climate-related studies. Understanding their variability in estimation is equally crucial for micro-meteorology and agricultural planning processes. The primary goal of this study was to analyze and compare estimates of (ET0) and (ET) from two different climatic regions of Kenya using long-term quality controlled synoptic station datasets from 2000 to 2009 with 3-hour time resolution. One weather station (Voi, 63793) was sought from lowlands with an elevation of 579 m and characterized by tropical savannah climate while the other (Kitale, 63661) was sought from Kenya highlands with humid conditions and elevation of 1850 m above sea level. Reference evapotranspiration was calculated based on the FAO 56 standard methodology of a daily basis. One dimension Palmer-type soil model was used for estimating of real evapotranspiration using the wilting point, field capacity, and soil saturation point for each station at 1 m deep soil layer. The ratio of real and reference evapotranspiration dependent on the soil moisture stress linearly. Calculations of estimated evapotranspiration were made on daily and monthly basis. Applications of the site-specific crop coefficients (KC) were also used. The result indicated that the differences among daily and monthly scale calculations of evapotranspiration (ET) were small without and with an application of crop coefficients (ETKc). This was due to high temperatures, global radiation, and also high soil moisture stress due to inadequate precipitation experienced in the tropics where Kenya lies. Results from Voi showed that mean monthly ET0 ranged from 148.3±11.6 mm in November to 175.3±10.8 mm in March while ET was from 8.0±4.5 mm in September to 105.8±50.3 mm in January. From Kitale, ET0 ranged from 121.5±8.5 mm/month in June to 157.1±8.5 mm/month in March while ET ranged from 41.7±32.6 mm/month in March to 126.6±12.2 mm/month in September. This was due to variability in temperature and precipitation between the two climatic regions. The study concludes that ET0 and calculated evapotranspiration variability among the years on a monthly scale is slightly higher in arid and semi-arid climate regions than in humid regions. The study is important in strategizing viable means to enhance optimal crop water use and reduce ET losses estimates for optimal agricultural yields and production maximization in Kenya.