Mean Actual Evapotranspiration Research Articles

THE WATER RELATIONS AND IRRIGATION REQUIREMENTS OF THE DATE PALM (Phoenix dactyliferaL.): A REVIEW

SUMMARYDate palm is a crop suited to hot, arid regions. It originated in Mesopotamia and the centres of production are in West Asia and North Africa. Despite its regional and international importance, and its dependence on irrigation or a shallow water table for survival, relatively little research has been published on the water relations and irrigation need of date palm. Following early work in California, the majority of the recent research reported in the literature has been conducted in Saudi Arabia and Tunisia. The date palm has a terminal crown of 100–120 leaves. At the base of each leaf is an axillary bud, most of which develop in the winter as flower buds. It takes 150–200 days from pollination to fruit maturity. Stomata occur on both leaf surfaces. Different techniques have been used to measure the water use of date palm, including micrometeorological and sap flow methods. In Syria, mean actual evapotranspiration rates varied between 0.5 mm d−1(winter) and 3.5 mm d−1(summer), in Saudi Arabia from 2–3 to 8–11 mm d−1depending on location and in Jordan from 2 to 8–10 mm d−1respectively. In Tunisia, there was some (limited) evidence of stomatal control of transpiration rates (seasonal range 0.5 to 3.5 mm d−1) when temperatures exceeded 32 °C. Experimentally determined values of the crop coefficient were inconsistent, varying from 0.6–0.7 to 1.18. In the vicinity of an oasis, advection may increase potential water use substantially above that of a reference crop. In the absence of a shallow water table, roots can extract water from soil depths greater than 2 m. There is little published information on the water productivity of date palm. A target benchmark figure is probably about 1.3-kg fresh fruit m−3of irrigation water applied. Under controlled conditions, phenotypes differed in their responses to water stress. The date palm is traditionally considered to be relatively salt-tolerant, with a threshold electrical conductivity value for the saturated soil extract of 4.0 dS m−1, but recent evidence from Israel suggests that this view may be mistaken. Since early times, flood irrigation has been used to irrigate date palm and it is still probably the most common method in many countries. Since the 1980s, farmers have been encouraged by governments to use localised irrigation methods (e.g. micro-sprinklers, drip and bubbler) as a means of saving water. Since water is a scarce resource in the West Asia and North Africa region, research should focus on developing ways to improve the water productivity of this high value crop.

Interspecific variation in primate coat colour supports Gloger’s rule

Aim In 1833, C.L. Gloger observed that bird populations living in warm and wet habitats were darker than those found in dry, cool areas. However, this hypothesis has seldom been evaluated, particularly for mammals. Here, we test Gloger’s rule using a dataset consisting of more than 100 primate species representing all major primate clades. Location Africa, Madagascar, Asia and the Neotropics. Methods We used museum skins, digital photography, and colour correction software to quantify the brightness of the dorsal and ventral pelage surface of each species. We utilized the mean actual evapotranspiration (AET) within the geographic range of each species as a proxy for habitat conditions and accounted for additional variables that may influence coloration. To analyse the data, we used a generalized linear model that simultaneously accounts for the effects of phylogenetic and spatial autocorrelation. Results We found that increasing levels of AET were significantly related to increasing pelage darkness on the dorsal surface of species, while accounting for other effects. Main conclusions Our finding provides further support for the applicability of Gloger’s rule to mammals, and is the first broad-scale evaluation for primates. The mechanism driving Gloger’s rule is not easy to discern, but may include increased background matching for species living in relatively light or dark habitats, increased resistance to keratin-degrading micro-organisms in hair with large amounts of eumelanin, and/or thermoregulation.

Estimation of actual evapotranspiration using the Surface Energy Balance System (SEBS) algorithm in the Upper Manyame catchment in Zimbabwe

Evapotranspiration plays a central role in the water balance of arid and semi-arid areas, as it is the major loss flux. The spatial estimation of actual evapotranspiration is thus of critical importance for hydrologic, environmental and agricultural purposes. This study therefore applied the Surface Energy Balance System for turbulent heat fluxes (SEBS) algorithm to determine actual evapotranspiration in the Upper Manyame catchment in Zimbabwe. The SEBS algorithm was run using atmospherically corrected MODIS satellite imagery on nine clear skies days stretching from 2003 to 2005. SEBS determined actual evapotranspiration was evaluated based on two criteria over Harare-Kutsaga and Grasslands research stations. Firstly, a plausibility or physical consistency check was applied by which SEBS actual evapotranspiration estimates were compared to FAO Penman–Monteith potential evapotranspiration estimates. On average, it was observed that SEBS actual evapotranspiration estimates were physically consistent. SEBS estimates were 36.5% and 76.5% of potential evapotranspiration over Harare-Kutsaga and Grasslands stations, respectively. However, on a few days over Grasslands station SEBS actual evapotranspiration estimates were physically inconsistent as they were greater than potential evapotranspiration. SEBS actual evapotranspiration estimates were also evaluated against actual evapotranspiration from the advection aridity equation. Over Harare-Kutsaga the mean error showed a slight underestimation of 0.32mmd−1 by SEBS and the mean absolute error was 0.5mmd−1. However, over Grasslands station the results were not as good as the mean error and mean absolute error showed an overestimation 2.6mmd−1. This was relatively high. The discussed reasons for the relatively high overestimation errors over Grasslands station include; roughness parameterisation, the spatial variability of input temperature data and heterogeneity. An analysis of the evaporative behaviour of the different land cover types in the catchment showed that: water bodies, closed broadleaved deciduous forests, and open broadleaved deciduous forests had relatively higher mean actual evapotranspiration rates and yet these land cover types constituted less than 5% of the catchment area. Open grasslands were observed to have low evaporative behaviour. Overall this study showed that SEBS has potential for estimating spatial actual evapotranspiration and providing useful information that can be used for water resources and environmental management and planning.

Scaling of potential evapotranspiration with MODIS data reproduces flux observations and catchment water balance observations across Australia

We developed a new algorithm for estimating monthly actual evapotranspiration (AET) based on surface reflectance from MODIS-Terra and interpolated climate data. The algorithm uses monthly values of the Enhanced Vegetation Index (EVI) and the Global Vegetation Moisture Index (GVMI) derived from the MODIS nadir bidirectional reflectance distribution function – adjusted reflectance product (MOD43B4) to scale Priestley-Taylor potential evapotranspiration derived from the climate surfaces. The EVI is associated with evapotranspiration through its relationship with leaf area index. The GVMI allows separation between surface water and bare soil when EVI is low and provides information on vegetation water content when EVI is high. The model was calibrated using observed AET data from seven sites in Australia, including two forests, two open savannas, a grassland, a floodplain and a lake. Model outputs were compared with four year average difference between precipitation and streamflow (a surrogate for mean AET) in 227 unimpaired catchments across Australia. We tested four different model configurations and found that the best results both in the calibration and evaluation datasets were obtained when a precipitation interception term ( E i ) and the GVMI were incorporated into the model. The E i term and the GVMI improved AET estimates in the forest, savanna and grassland sites and in the lake and floodplain sites respectively. The most comprehensive model estimated monthly AET at the seven calibration sites with a RMSE of 18.0 mm mo −1 (22% of the mean AET, r 2 = 0.84). In the evaluation dataset, mean annual AET was estimated with a RMSE of 137.44 mm y −1 (19% of the mean AET, r 2 = 0.61). The model was able to reproduce the main spatial and temporal patterns in AET across Australia. The main advantages of the proposed model are that it uses a single set of parameters (i.e. does not need an auxiliary land cover map) and that it is able to estimate AET in areas with significant direct evaporation, including lakes and floodplains.

Groundwater Recharge Assessment for the Kalahari Catchment of North-eastern Namibia and North-western Botswana with a Regional-scale Water Balance Model

Groundwater is the only source of drinking water for the inhabitants of the Kalahari. Thus understanding spatial and temporal variations in groundwater recharge is very important and a regional-scale water balance model has therefore been set up for a 209,149 km2 catchment in north-eastern Namibia and north-western Botswana. The model has a spatial resolution of 1.5 × 1.5 km, daily model time-steps, and climatic input parameters for 19 years are used. The distributed, GIS-based, process-oriented, physical water balance model (MODBIL) used in this study considers the major water balance components: precipitation, evapotranspiration, groundwater recharge, and surface runoff/interflow. Mean precipitation for the study area is 409 mm a−1, while mean actual evapotranspiration is 402 mm a−1 and mean groundwater recharge is 8 mm a−1 (2% of mean annual precipitation). The recharge pattern is mainly influenced by the distribution of soil and vegetation units. Groundwater recharge shows a high inter- and intra-annual variability, but not only the sum of annual precipitation is important for the development of groundwater recharge; a large amount of precipitation in a relatively short period is more important. Published independent data from the Kalahari in Namibia, Botswana and the Southern African region under similar climatic conditions are used to verify the modelling results.

Tree diversity in the northern Neotropics: regional patterns in highly diverse Chiapas, Mexico

Physical factors that may account for regional patterns of plant species diversity remain controversial. We aim to determine the relationship of tree species diversity to environmental factors identifiable at regional scale in the northern Neotropics. We use a high‐resolution data set based on herbarium collections of all native tree species known to occur in the highly diverse and physiographically heterogeneous Mexican state of Chiapas. We analyzed 114 grid cells (5 min latitude×5 min longitude each) with 40 or more vouchers. We obtained from maps (scale 1:250 000) data on temperature, rainfall, elevation, and soils, and calculated for each grid cell mean actual evapotranspiration (AET), its ratio during the rainy and dry seasons (RET), average fertility/quality of soils (SFQ), and elevation (coarse‐scale topography) variance (SDE). These variables were largely independent of each other, and were entered in multiple regression models to predict species diversity assessed with Simpson's index of concentration. A model that accounted for 41.4% of the total variance in tree diversity showed positive effects of AET and seasonality (RET), whereas SFQ had a negative effect. A curvilinear model described well the relationship between tree diversity and AET (R2=0.45), and an intermediate maximum was detected. The data pattern suggested an asymptotic relationship as well, which was confirmed with a two‐part regression. Regression quantiles provided better estimates of the effect of SFQ with the upper envelope of the data (0.85–0.90 quantiles). Minimum diversity at intermediate rainfall values hints at a bimodal model of tree diversity along a rainfall gradient, in opposition to the frequent contention of a positive linear relationship. We suggest that broad‐scale climatic gradients interact with intraregional landscape‐level influences, thus leading to the observed nonlinear responses of tree diversity to environmental predictors.

A simple surface energy balance to derive evapotranspiration from remote sensing and conventional meteorological observations

Monthly mean measurements of surface temperature, albedo and normalized difference vegetation index (NDVI) of NOAA AVHRR are processed for Maharashtra. These data are used in combination with monthly average surface meteorological observations in a surface energy balance model to estimate monthly mean actual evapotranspiration (AET) from different climatic zones of Maharasthra,. India. AET is estimated between April and December months for two contrasting monsoons in 1992 and 1995. Estimates reasonably agree with pan evaporation data during growing season and with AET estimated from water balance procedure. AET is low in semi-arid dry land areas of central Maharashtra and significantly high in the humid-perhumid western ghat region and subhumid eastern Maharashtra region. The modeled evapotranspiration show the influence of seasonal vegetation in different climatic zones from the region. The method can be used to obtain large-scale evapotranspiration with minimum ground observations.

Climate, Leaf Litter Chemistry and Leaf Litter Decomposition in Terrestrial Ecosystems: A Triangular Relationship

Litter decomposition is an important component of the global carbon budget. Due to the strong climatic control of litter decomposition, climate change may significantly affect this pathway. This review quantifies the climatic influences on litter decomposition rates, both directly and indirectly through effects on litter chemistry. To this end, I analysed first-year leaf litter decomposition data from 44 locations, ranging from cool temperate sites to humid tropical sites. Actual evapotranspiration (AET) was used as an index for the climatic control on decomposition. As litter chemistry parameters I included N and P concentrations, C/N and C/P ratios, lignin concentrations, and lignin/N and lignin/P ratios. At a global scale, climate (expressed as AET) is the best predictor for the decomposition constants (k-values) of the litter, whereas litter chemistry parameters have much lower predictive values. Path analysis showed that the control of AET on litter decomposability is partly mediated through an indirect effect of AET on litter chemistry. Thus, the relation between climate, leaf litter chemistry and leaf litter decomposition is a triangular relationship. Mean AET in the humid tropical region is three times as high as in both the temperate and the Mediterranean region and this results in a more than six-fold increase in mean k-values. However, due to the large variability in k-values within each region there is a considerable overlap in k-values between the tropics and the other climatic regions. Within a particular climatic region litter chemistry parameters are the best predictors of k-values, especially in the tropics, whereas the percentage of variance in k-values explained by AET is low or absent. In general, litters from the tropical sites have higher N concentrations and lower lignin/N ratios than litters from other climatic regions. In both the tropics and in the Mediterranean region, the lignin/N ratio is the best chemical predictor of litter decomposability. In the temperate region, however, there is no good chemical predictor of litter decomposability.

Temporal Variation in Actual Evapotranspiration of Terrestrial Ecosystems: Patterns and Ecological Implications

We compared the water balance among the earth's major terrestrial ecosystems. Potential evapotranspiration, actual evapotranspiration (AET) and deficit were derived for each month and year from 19-25-yr climate records at ninetyfour sites around the world representing eleven biomes. For each variable, we determined mean annual values. Our focus, however, was to examine temporal variation in AET, which previously has been correlated with large-scale patterns of ecosystem structure and function. Standard deviation of annual AET, an absolute measure of interannual variability, was highest for grasslands and lowest for tundra and taiga. Coefficient of variation of annual AET, a relative measure of variability, was negatively related to mean AET, and was higher for non-forested than for forested ecosystems. Monthly variation, an index of seasonality, and interannual variation of AET were positively related for forested ecosystems and negatively related for non-forested ecosystems. There was a positive relationship between interannual variability and variation among sites within a biome. Results indicate (1) timescale dependent differences in climatic variability among biomes that may help predict functional properties and explain structural patterns among terrestrial ecosystems; and (2) a link between temporal variability and spatial heterogeneity among