Direct Measurement Of Evapotranspiration Research Articles

Estimating crop evapotranspiration of wheat-maize rotation system using hybrid convolutional bidirectional Long Short-Term Memory network with grey wolf algorithm in Chinese Loess Plateau region

Accurate estimation of crop evapotranspiration (ET) is essential for the efficient utilization of agricultural water resources, crop production enhancement, and sustainable agricultural development. However, direct measurement of ET is highly expensive, intricate, and time-consuming, highlighting the imperative of establishing a novel model to accurately estimate ET in agricultural ecosystems. To address the above problems, this study proposed a novel model (GWA-CNN-BiLSTM), which incorporates Grey Wolf Algorithm (GWA), Convolutional Neural Network (CNN), and Bidirectional Long Short-Term Memory network (BiLSTM) as a hyperparameter adjuster, feature extractor, and regression component, respectively, to estimate ET built upon various input combinations comprising net solar radiation (Rn), vapor pressure deficit (VPD), average air temperature (Ta), soil water content (SWC), and leaf area index (LAI) about winter wheat-spring maize rotation system during 2012–2020 in the Loess Plateau. Besides, following a comparative assessment within GWA-CNN-BiLSTM, Convolutional Bidirectional Long Short-Term Memory network (CNN-BiLSTM), BiLSTM, Long Short-Term Memory network (LSTM), and Shuttleworth-Wallace (SW) models, the results revealed that GWA-CNN-BiLSTM under varied inputs obtained the superior performance, ranging from 0.562 to 0.957 in determination coefficient (R2), 8.4–41.5 % in relative root mean square error (RRMSE), 0.349 mm d−1 to 1.521 mm d−1 in mean absolute error (MAE), −3.26 % to 14.11 % in percent bias (PBIAS), and 0.820–1.091 in regression coefficient (b0), respectively. Moreover, while the accuracy of BiLSTM over LSTM was evident, its performance was notably improved by the incorporation of the CNN module. Additionally, LSTM-type models under complete input combination present better precision than SW by 29.7−51.4 % in R2, 44.2−76.1 % in RRMSE, and 33.6−63.4 % in MAE, respectively. Furthermore, the accuracy of all models under varied inputs exhibited excellence in winter wheat compared to spring maize, and corresponding improvements ranged 1.4−4.3 % in R2, 5.1−20.1 % in RRMSE, and 3.1−17.9 % in MAE, respectively. Besides, the meteorological factors (Rn, VPD, Ta) proved to be the most important inputs for ET estimation in winter wheat and spring maize. Wherein the importance of SWC exceeded that of LAI in winter wheat, while the opposite trend was observed in spring maize. In brief, GWA-CNN-BiLSTM is the highly recommended model to estimate ET of winter wheat-spring maize rotation system under diverse input data scenarios in the Loess Plateau, which can facilitate to offer valuable assistance in regional agriculture water management decisions.

Modeling monthly actual evapotranspiration: an application of geographically weighted regression technique in the Passaic River Basin

Abstract Actual evapotranspiration (ET) is perhaps the most difficult quantity to directly measure among the major water balance components. Because of the high cost and labor constraints associated with the direct measurement of ET, empirical data-driven modeling has frequently been used to estimate ET. Beyond the widely used traditional type regression that has the effect of producing ‘global’ parameter estimates, assumed to be uniform throughout an area, we utilized a more localized spatially non-stationary technique – the geographically weighted regression (GWR) – to estimate mean monthly ET in the Passaic River Basin (PRB). We identified the key environmental controls of ET and developed new sets of spatially varying empirical ET models based on variable combinations that produced the best-fit model. The analysis showed that temporal and spatial variabilities in ET over the PRB are driven by climatic and biophysical factors. We found that the key controlling factors were different from month to month, with wind speed being dominant throughout the year in the study basin. A monthly mean ET index map was further generated from the model to illustrate areas where ET exceeds precipitation. This will among others enable water loss due to evapotranspiration to be accounted for in future water supply plans for the basin.

Monitoring the hourly actual evapotranspiration through the energy balance and a weighing lysimeter

By having sufficient knowledge about evapotranspiration in space and time, it is possible to manage and plan the water management of a basin. Evapotranspiration can be obtained indirectly at a local scale through information recorded in agrometeorological stations, or regionally with the use of satellite data. In order to have reliable models, it is very important to validate the information generated with direct measures of evapotranspiration.Therefore, direct measurements of evapotranspiration in clay secano-soils covered by grasses and the use of the data generated for the validation of indirect measurements are presented in this study. To achieve this, hourly data from an energy balance station and direct measurements of a weighing lysimeter were used to study the water loss during two experimental campaigns carried out in Tandil (Argentina), in the months February-March and September 2017. The comparison of the methods was made for different moisture conditions of the soil. The energy balance model estimates approximate values of actual evapotranspiration for maximum values of soil moisture (0.01-0.15 mm hour-1), showing greater differences (0.1-0.35 mm hour-1) when the soil moisture tends to minimum values. However, it was demonstrated that a simple one-level model to determine the latter variable is applicable for areas with reference coverage, allowing simplifications in aerodynamic and crop resistance measurements (Allen et al. 1998). This contribution provides necessary information for the validation of satellite models applied to plains areas, where evapotranspiration is a term of great importance.

Modelling water utilization patterns in apple orchards with varying canopy sizes and different growth stages in semi-arid environments

Accurate estimates of orchard evapotranspiration (ET) and its components are important for precise irrigation scheduling, irrigation system designs, and optimal on-farm water allocation particularly in water-limited environments. Direct measurements of ET remain costly, laborious and sometimes difficult to apply over heterogeneous surfaces such as crop fields. Therefore, accurate crop water-use models are required for on-farm precise water resources management. In this study, we adopted and improved the Priestley-Taylor Jet Propulsion Laboratory (PT-JPL) model developed by Fisher et al 2008 to estimate crop water use across different apple plants. Specifically, the model was developed to quantify the partitioning of apple orchard water use into beneficial (tree transpiration) and non-beneficial water use (orchard floor evaporation) as influenced by tree canopy cover. Data were collected in twelve orchards spread across key apple producing regions in the Western Cape Province of South Africa over three growing seasons (2014/15, 2015/16, 2016/17). Model ET estimates were tested against ET data measured; using the eddy covariance method and transpiration measured based on sap flow monitoring techniques. The results showed that the original Fisher PT-JPL model performed poorly in ET estimation across all the orchards under study. The model yielded lower R2, ranging from 0.02 to 0.64 and Nash-Sutcliffe Efficiency (NSE) from -10.93 to 0.20. Thus, we subsequently improved the model by incorporating soil moisture and vapour pressure deficit stress factors and by introducing a variable Priestley and Taylor coefficient (α). The modified PT-JPL model demonstrated an improvement in ET estimates. The root mean square of error (RMSE) of the estimated daily ET varied from ±0.60 mm/d to ±1.99 mm/d whereas the mean absolute error (MAE) varied from ±0.49 mm/d to ±1.91 mm/d, while the R2 varied from 0.54 to 0.75 in orchards with varying canopy cover. The findings of this work underscore the utility of the modified PT-JPL model for estimating ET and its components in apple orchards from planting until the trees reach full-bearing age.

Light Quality Affects Water Use of Sweet Basil by Changing Its Stomatal Development

Different light qualities affect plant growth and physiological responses, including stomatal openings. However, most researchers have focused on stomatal responses to red and blue light only, and the direct measurement of evapotranspiration has not been examined. Therefore, we quantified the evapotranspiration of sweet basil under various red (R), green (G), and blue (B) combinations using light-emitting diodes (LEDs) and investigated its stomatal responses. Seedlings were subjected to five different spectral treatments for two weeks at a photosynthetic photon flux density of 200 µmol m−2 s−1. The ratios of the RGB light intensities were as follows: R 100% (R100), R:G = 75:25 (R75G25), R:B = 75:25 (R75B25), R:G:B = 60:20:20 (R60G20B20), and R:G:B = 31:42:27 (R31G42B27). During the experiment, the evapotranspiration of the plants was measured using load cells. Although there were no significant differences in growth parameters among the treatments, the photosynthetic rate and stomatal conductance were higher in plants grown under blue LEDs (R75B25, R60G20B20, and R31G42B27) than in the R100 treatment. The amount of water used was different among the treatments (663.5, 726.5, 728.7, 778.0, and 782.1 mL for the R100, R75G25, R60G20B20, R75B25, and R31G42B27 treatments, respectively). The stomatal density was correlated with the blue light intensity (p = 0.0024) and with the combined intensity of green and blue light (p = 0.0029); therefore, green light was considered to promote the stomatal development of plants together with blue light. Overall, different light qualities affected the water use of plants by regulating stomatal conductance, including changes in stomatal density.

Multi-Scale Evaluation of the TSEB Model over a Complex Agricultural Landscape in Morocco

An accurate assessment of evapotranspiration (ET) is crucially needed at the basin scale for studying the hydrological processes and water balance especially from upstream to downstream. In the mountains, this term is poorly understood because of various challenges, including the vegetation complexity, plant diversity, lack of available data and because the in situ direct measurement of ET is difficult in complex terrain. The main objective of this work was to investigate the potential of a Two-Source-Energy-Balance model (TSEB) driven by the Landsat and MODIS data for estimating ET over a complex mountain region. The complexity is associated with the type of the vegetation canopy as well as the changes in topography. For validating purposes, a large-aperture scintillometer (LAS) was set up over a heterogeneous transect of about 1.4 km to measure sensible (H) and latent heat (LE) fluxes. Additionally, two towers of eddy covariance (EC) systems were installed along the LAS transect. First, the model was tested at the local scale against the EC measurements using multi-scale remote sensing (MODIS and Landsat) inputs at the satellite overpasses. The obtained averaged values of the root mean square error (RMSE) and correlation coefficient (R) were about 72.4 Wm−2 and 0.79 and 82.0 Wm−2 and 0.52 for Landsat and MODIS data, respectively. Secondly, the potential of the TSEB model for evaluating the latent heat fluxes at large scale was investigated by aggregating the derived parameters from both satellites based on the LAS footprint. As for the local scale, the comparison of the latent heat fluxes simulated by TSEB driven by Landsat data performed well against those measured by the LAS (R = 0.69, RMSE = 68.0 Wm−2), while slightly more scattering was observed when MODIS products were used (R = 0.38, RMSE = 99.8 Wm−2). Based on the obtained results, it can be concluded that (1) the TSEB model can be fairly used to estimate the evapotranspiration over the mountain regions; and (2) medium- to high-resolution inputs are a better option than coarse-resolution products for describing this kind of complex terrain.

Analysis of Dynamic Spatiotemporal Changes in Actual Evapotranspiration and Its Associated Factors in the Pearl River Basin Based on MOD16

Evapotranspiration is an important part of the hydrological cycle, surface energy balance and global climate system. Due to spatial heterogeneity, the trends in actual evapotranspiration (ET) and its associated factors vary in different regions. Because direct measurements of ET are limited over large areas, remote sensing provides an efficient method of ET spatial analysis, and standard data products are available at the global scale. This study uses the monthly MOD16 ET dataset and daily meteorological data to analyze the dynamic spatiotemporal changes in ET and its associated factors in the Pearl River Basin (PRB) from 2000 to 2014. The results of the study are as follows. (1) Over time and space, annual ET exhibited a slight increasing trend from 2000 to 2014, with an average value of approximately 946.56 mm/a. ET considerably varied at the monthly and seasonal scales, and in July displayed the highest monthly ET of approximately 119.57 mm, accounting for 36.37% of the annual ET. (2) ET displayed obvious spatial heterogeneity. Specifically, the west was a low-ET region, and moderate and high ET values were interspersed in the central and eastern PRB. Moreover, the rate of change of ET ranged from −13.99 mm/a to 12.81 mm/a in space, and 46.25% of the basin exhibited an increasing trend. (3) Dynamic changes in ET were mainly associated with temperature and relative humidity (RH). Additionally, energy-related elements and wind speed were positively correlated with ET, and temperature was the most influential factor of ET in some months (February–March and September–November). RH was the most important factor in other months but negatively correlated with ET in June and July. Affected by the actual environmental condition, qualitative changes were observed in the correlation between RH and ET in different months. The positive and negative spatial correlations between ET and its associated factors changed in different regions and in different months, and the changes mainly occurred from northwest to southwest.

Influence of Growth of Reeds on Evapotranspiration in Horizontal Subsurface Flow Constructed Wetlands

In this study, the influence of growth of reeds on evapotranspiration (ET) was estimated, and a commonly used meteorological estimate of potential evapotranspiration (PET) was compared with direct measurements of ET. The salinity of the inside of HSF of the raw leachate inflow was 15.0±3.4g Cl-/L and that of the double diluted inflow was 9.3±1.9g Cl-/L. Although the growth of reeds in the raw leachate inflow was impeded remarkably compared to that of the double diluted leachate inflow, the reeds in the double diluted inflow bed showed healthy growth. The difference in the salinity gave rise to large differences in the growth of the reed. The annual ET rates in the poor vegetation bed, the dense vegetation bed and the unplanted bed were 656.5mm, 2,334.3mm and 22.2mm, respectively. The difference of the growth of the reed provided a large difference in the ET rate. The annual PET estimated on the basis of the Hamon equation was 751.6mm. The PET rate was much lower compared to the ET rate in the dense vegetation bed. It was necessary to consider site-specific factors such as the growth of plants in the evaluation of the water budget in the HSFs.

Direct measurement of evapotranspiration from a forest using a superconducting gravimeter

AbstractEvapotranspiration (ET) controls the flux between the land surface and the atmosphere. Assessing the ET ecosystems remains a key challenge in hydrology. We have found that the ET water mass loss can be directly inferred from continuous gravity measurements: as water evaporates and transpires from terrestrial ecosystems, the mass distribution of water decreases, changing the gravity field. Using continuous superconducting gravity measurements, we were able to identify daily gravity changes at the level of, or smaller than, 10−9 nm s−2 (or 10−10 g) per day. This corresponds to 1.7 mm of water over an area of 50 ha. The strength of this method is its ability to enable a direct, traceable and continuous monitoring of actual ET for years at the mesoscale with a high accuracy.

Post-Processing Techniques for Reducing Errors in Weighing Lysimeter Evapotranspiration (ET) Datasets

Abstract. In agriculture, evapotranspiration (ET) is a major consumptive use of irrigation water and precipitation on cropland. Global interest in sustainable management of limited freshwater supplies to meet increased food demand has resulted in increased reporting of ET measurement and modeling methods in the literature. Direct measurements of ET by large weighing lysimeters are commonly used to test other ET measurement and estimation methods. Numerous studies have emphasized the importance of proper lysimeter design and management for accurate ET measurement. However, equally important and noticeably absent from the literature are guidelines for data collection, processing, analysis, and quality assurance and quality control (QA/QC) measures. Omission and/or inadequate attention to these processes can have significant impacts on the accuracy of ET derived from large weighing lysimeters. Improper processing of rainfall, irrigation, snowfall, dew and frost accumulation, wind, and management events can also lead to substantial errors. In this study, we present data processing strategies and techniques for ET datasets measured using large weighing lysimeters. Measurements from two large lysimeters built and managed by the USDA-ARS Conservation and Production Laboratory (CPRL) in Bushland, Texas, were used for this purpose. Examples used in the study demonstrate that indiscriminate application of smoothing functions, misidentification, and misinterpretation of changes in lysimeter mass can lead to significant errors and erroneous conclusions. Comprehensive record keeping is paramount in documenting lysimeter status and operations. Errors associated with data processing are dataset specific, corresponding to the magnitude, duration, and frequency of occurrences of the aforementioned events. Understanding and prudent application of the presented techniques and QA/QC procedures can help to minimize errors in processing lysimeter datasets.

Characteristics of Reference Evapotranspiration of Wetland Based on Penman-Monteith and Hargreaves Methods

Evapotranspiration (ET) constitutes the dominant water loss from many different types of wetlands. The relative importance of ET is apparent in its influence over water depth, temperature and salinity. However, direct measurement of ET, especially for wetland, is difficult, costly, and rarely available. In this study, the Penman-Monteith model was selected to estimate the reference evapotranspiration for short and tall canopies, moreover, Hargreaves equation was also used to simulate the ETref and test the precision of Penman-Monteith method. The results show that the reference evapotranspiration are maximal between June and August and minimal in winter, and ETrs is larger than ETos in the whole simulation periods. H ET0 and ETos have the same variation trend, but the former was smaller than the latter during May to September. Finally, there is a strong correlation between ETos and ETrs, with the correlation coefficient are 0.98 and 0.99 at daily and monthly means scale.

On Estimating Wet Canopy Evaporation from Deciduous and Coniferous Forests in the Asian Monsoon Climate

Abstract Continuous and direct measurement of evapotranspiration (ET) by the eddy covariance (EC) technique is still a challenge under monsoon climate because of a considerable amount of missing data during the long rainy periods and the consequential gap-filling process. Under such wet canopy conditions, especially in forests, evaporation of the intercepted precipitation (EWC) contributes significantly to the total ET. To quantify the role of EWC, leaf wetness has been measured at multiple levels in the canopy simultaneously with eddy covariance measurements at the KoFlux Gwangneung deciduous and coniferous forests for the entire year from September 2007 to August 2008. In this study, the measured EWC and the controlling mechanism during the wet canopy conditions have been scrutinized. Based on the evaluation of the four different algorithms of EWC estimation, that of the variable infiltration capacity (VIC) land surface model (LSM) has been adopted. All the missing EWC data are then recalculated by using the algorithm of VIC LSM and compared against the traditionally gap-filled EWC data based on the modified lookup table (MLT) method. The latter consistently underestimated EWC on average by 39% in deciduous forest and by 28% in coniferous forest. Major causes of such differences were due to the failure of considering aerodynamic coupling, advection of sensible heat, and heat storage in the MLT-based gap-filling method. Accordingly, a new gap-filling strategy for EWC is proposed that takes proper controlling mechanisms into account.

First long-term, direct measurements of evapotranspiration and surface water balance in the Nebraska SandHills

The Nebraska SandHills is a vast grassland ecoregion that also serves as the main recharge zone for the High Plains (Ogallala) aquifer. Despite the national importance of this region in supplying irrigation water and supporting the US cattle industry, there have, until now, been no long-term, direct measurements of the components of the surface water balance. We have addressed this issue by installing energy balance/Bowen ratio (EBBR) flux towers in three key ecosystems. We report here the results of 7 years of measurements and interpret them in terms of annual weather variability. Additionally, these data have allowed us to make first-order estimates of the recharge rate to the aquifer. We found that the three ecosystems behave in very different ways, and that, in a first-order estimate, the region provides approximately 115±20mm year−1 of recharge to the aquifer over the 2003–2009 period.

EFEITO DE LÂMINAS DE IRRIGAÇÃO SOBRE O CRESCIMENTO DO ALGODOEIRO HERBÁCEO

The work aimed at studying the effect of irrigation levels on the herbaceous cotton growth. To this end, an experiment was conducted under controlled conditions, protected environment, screenhouse, in the Mossoro city, Rio Grande do Norte State, Brazil, between June and October 2006, a column of soil from 40 cm in height and diameter of 15 cm. It was used to completely randomized design and the cultivars (CNPA 8H and BRS Ruby) and the irrigation levels (55%, 70%, 85%, 100% and 115% of ETc) arranged in factorial scheme 2 x 5, with four repetitions. The levels were determined from the direct measurement of evapotranspiration, obtained by weighing vessel received water total replacement. The data on growth (plant height, stalk diameter and number of leaves per plant) were taken to 130 days after planting, or at the time of harvest. There are differences between the cultivars on the stalk diameter. The models of best fit the water levels applied followed linear trend of height plants and quadratic in diameter and number of leaves

An Automated Rotating Lysimeter System for Greenhouse Evapotranspiration Studies

Lysimeters are important tools in soil–plant–atmosphere research because they allow direct measurements of evapotranspiration and facilitate studies of water, nutrient, and solute balances. Heterogeneity in the atmospheric conditions in greenhouses causes spatial variations in evapotranspiration. Accurate design and management of water balance research in greenhouses is difficult due to restraints of replication and placement needed to statistically minimize the influence of heterogeneity. Precision and control are necessary to maintain precise boundary conditions, a prerequisite for accurate investigations of selected variables. We present a rotating structure that significantly enhances uniformity in environmental conditions between individual free‐standing lysimeters placed on the structure. The system contains automatic irrigation (water and fertilizer) delivery and drainage collection devices. The system was tested in evaporation and plant growth experiments. A dual carousel setup was used to measure evaporation from containers placed on the soil surface of the lysimeters, as well as yield of lettuce (Lactuca sativa L.) plants growing in the lysimeters. Half of each carousel was covered with netting to reduce solar radiation by 50%. One carousel rotated throughout the experiment while the other one remained stationary. Variability in measured data was reduced as a result of rotation. The CV of measured evaporation from the rotating system (3.3%) was significantly lower than that of the stationary system (16.2%). Similarly, the CV of relative lettuce yields of the rotating system was 11.9% as compared with 20.9% for the stationary system. The proposed apparatus is very attractive as a research and teaching tool for studying plant physiological processes, flow and transport in soils, and plant response to management or environmental variables.

Estimating evapotranspiration in natural and constructed wetlands

Difficulties in accurately calculating evapotranspiration (ET) in wetlands can lead to inaccurate water balances—information important for many compensatory mitigation projects. Simple meteorological methods or off-site ET data often are used to estimate ET, but these approaches do not include potentially important site-specific factors such as plant community, root-zone water levels, and soil properties. The objective of this study was to compare a commonly used meterological estimate of potential evapotranspiration (PET) with direct measurements of ET (lysimeters and water-table fluctuations) and small-scale root-zone geochemistry in a natural and constructed wetland system. Unlike what has been commonly noted, the results of the study demonstrated that the commonly used Penman combination method of estimating PET underestimated the ET that was measured directly in the natural wetland over most of the growing season. This result is likely due to surface heterogeneity and related roughness efffects not included in the simple PET estimate. The meterological method more closely approximated season-long measured ET rates in the constructed wetland but may overestimate the ET rate late in the growing season. ET rates also were temporally variable in wetlands over a range of time scales because they can be influenced by the relation of the water table to the root zone and the timing of plant senescence. Small-scale geochemical sampling of the shallow root zone was able to provide an independent evaluation of ET rates, supporting the identification of higher ET rates in the natural wetlands and differences in temporal ET rates due to the timing of senescence. These discrepancies illustrate potential problems with extrapolating off-site estimates of ET or single measurements of ET from a site over space or time.

Fog deposition to Norway Spruce stands at high-elevation sites in the Eastern Erzgebirge (Germany)

Fog deposition was quantified by comparing the water budget of a small head-water catchment with direct measurements of evapotranspiration by the eddy-covariance-energy-budget-(ECEB)-method and with results of a resistance model. Results from the model showed a distinct altitudinal gradient in fog deposition mirroring different fog frequencies. For two water budget years the model amounted to 270 mm of occult precipitation, while the water budget comparison reached 215 mm. To characterize its contribution to element deposition fog was collected by active and passive samplers. Element fluxes were calculated by multiplying concentrations with modeled water fluxes stratified by wind directions in order to account for the influence of the Bohemian Industrial Basin. Element deposition through fog was in the case of SO 4 2−–S, NH 4 +–N and K + higher than wet deposition. Fog deposition is so an important path within bulk deposition for forested hill sites. The measured ion concentrations in fog water have declined considerably (60–90%) over the last 10 years at the same site. This trend confirms with other measurements of air pollution and atmospheric deposition in the Eastern Erzgebirge.

Measurement of Evapo-Transpiration from Large Tanks of Soil

THE direct measurement of evapo-transpiration on any sufficiently large scale requires very expensive equipment. In an attempt to find a cheaper method we have been experimenting with semi-enclosed hydraulic systems involving the use of water-filled, flexible bags on which large tanks can be readily supported (Fig. 1). The change in height of the balancing column of water measures changes in pressure on the system.

Measuring Transpiration of Undisturbed Tamarisk Shrubs

Scarcity of water suitable for human use has influenced the history of mankind profoundly (4) and is now of major concern in the United States. It is of acute local concern in Phoenix, Ariz. and desert environs, where a rapidly increasing human population depends on one remote watershed (Salt River) for its primary water supply (2). Here, problems of watershed management are not of mere scholarly interest; they are of intense and immediate public concern. One of the major problems is how to reduce evaporative losses from the watershed itself. This is a general problem in watershed management (6), and its importance increases with aridity of a region. On one experimental part of the Salt River watershed, evaporative losses of summer rainfall ranged from 89 to 100% (12). Evaporative losses from soil and vegetation, usually considered together as evapotranspiration (ET), can be changed by altering vegetation (6). Much more information is needed about what kind, size, age, and arrangement of plant cover is least wasteful for specific situations and there is acute need for a convenient method for direct measurement of ET in undisturbed situations (6). In the present paper is reported the development of one such method and its use on thickets of fivestamen tamarisk (Tamarix pentandra Pall). Tamarisk, also known as saltcedar (7), is an important phreatophyte along the Salt River and throughout much of the Southwest. Replacement of tamarisk by plants that transpire less has been suggested as a means of reducing ET (13). Bermuda grass [Cynodon dactylon (L) Pers] may be a feasible replacement along the lower stretches of the Salt River (8).

The Climate of South Africa According to Thornthwaite's Rational Classification

In this paper the climate of South Africa is classified along the lines of C. W. Thornthwaite's paper “An Approach Toward a Rational Classification of Climate” which appears in the American Geographical Review of January 1948. Since only the salient points of this classification together with examples are reiterated here, It is advisable for the reader to have recourse to the said paper for a more complete understanding of the subject. The maps, especially as regards the mountainous regions, are necessarly only a generalised version of the true picture that would result from an evenly distributed and close network of stations. In the second part of this paper an account is given of an attempt at the direct measurement of evapotranspiration under natural high-veld conditions. It is hoped that the results obtained, though not conclusive, may stimulate further research in this particular aspect of climatology.