Spatial Variation of Reference Evapotranspiration and its Influence on the Hydrology of Luvuvhu River Catchment, Limpopo Province, South Africa

Table of Contents

Abstract

1. Introduction

2. The Study Area

3. Material and Methods
3.1 Hydrometeorological Data
3.2 Evapotranspiration Analysis
3.3 Statistical Analysis

4. Results
4.1 Hydrometeorological Analysis
4.2 Reference Evapotranspiration Analysis
4.3 Statistical Analysis

5. Conclusions

References

SPATIAL VARIATION OF REFERENCE EVAPOTRANSPIRATION AND ITS INFLUENCE ON THE HYDROLOGY OF LUVUVHU RIVER CATCHMENT, LIMPOPO PROVINCE, SOUTH AFRICA

L.R. SINGO¹, P.M. KUNDU¹, J.O. ODIYO¹ & F.I. MATHIVHA¹

ABSTRACT

Evapotranspiration (ET) is regarded as the largest mode of water loss in arid and semi- arid areas and is critical for accurate predictions of water exchange and crop productivity. Analysis of spatial and temporal fluctuations of evapotranspiration is therefore crucial to understand the coupled water and energy cycles in arid and semi-arid environments. This study estimated reference evapotranspiration (ETo) to study its influence on the hydrology of Luvuvhu River Catchment using a physically based model. The ETo plays a key role in irrigation systems design, water management under irrigated and rainfed production. The Penman-Monteith equation which is widely used in water resource management and planning was used to estimate ETo. Simulation of ETo was performed using CROPWAT 8.0 software. This algorithm is useful in simulating water resource management scenarios at different spatial and temporal scales under a wide range of environmental conditions. To understand the impact of climatic characteristics in the formulation of evapotranspiration model, seasonal variation of different meteorological parameters such as wind speed, solar radiation, temperature and humidity was analysed. Results showed the spatial and temporal distribution of ETo for different climatic stations in the study area with peaks in summer months. Minimum values of ETo were observed during the dry months. Results from the simulations showed that the areas with higher ETo values were near rivers and streams, which generally have more abundant vegetation. Areas with low ETo values were relatively dry, where pasture and grasslands dominated the landscape. Correlation results showed that no relationship exists between stream flow and ETo (r = 0.36) in the study area, hence, a significant relationship exists between rainfall and ETo (r = 0.86). The study recommends the use of CROPWAT model for computing ETo under arid and semi-arid climatic conditions for water resource management and planning.

Keywords: CROPWAT, catchment, Penman-Monteith, reference evapotranspiration, simulation.

1 INTRODUCTION

Evapotranspiration (ET) is the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere. The ET is influenced by local conditions that range from precipitation and meteorology to soil moisture, plant water requirements and the physical nature of the land cover (Abdullahi¹ ). Hence, ET is a key process within the hydrological cycle and possibly the most difficult component to determine, especially in arid and semi-arid areas where a large proportion of low and sporadic precipitation is returned to the atmosphere via evapotranspiration (Jovanovic et al.² ). In these areas, ET is roughly equal in magnitude to precipitation on timescales longer than seasons (Reynolds et al. ³ ); and is regarded as the largest mode of water loss critical for accurate predictions of water exchange and crop productivity (Moiwo et al.⁴ ). Knowledge of ET is therefore critical for sustainable water resources management and balanced water supply among industrial, domestic, ecological, and agricultural sectors (Moiwo et al.⁵ ). Whereas climate and hydrological variables such as precipitation and runoff are measured with reasonable accuracy, ET is difficult to quantify especially at the basin/regional scale. Estimates of ET are therefore required in the design of reservoirs, irrigation systems, scheduling and frequency of irrigation and water balance and simulation studies (Moiwo et al.⁵ ).

In cultivated semi-arid regions such as the Luvuvhu River Catchment (LRC) where rainfall occurs on limited basis, estimation of ET is crucial for management of water resources. As a result of spatial variability of landscape characteristics such as topography, soils, land use/cover, and vegetation, ET will likely exhibit a spatial variation in semi-arid catchments (Güntner & Bronstert ⁶ ). In these areas, measurements of ET are rarely available. Evaporation pan coefficients from Symon’s pan and Class-A pan have been used by the Department Water Affairs and Sanitation (DWAS) and South African Weather Services (SAWS), respectively, to estimate total annual evaporation losses in the study area. These methods are often not reliable for estimating the seasonal variation in evaporation losses due to heat storage in deep lakes. The DWAS⁷ estimated that the highest evaporation (about 60%) occurs during rainfall season while lowest evaporation (about 40%) occurs in dry season. The high evaporation significantly reduces effective rainfall, runoff, soil infiltration, groundwater recharge and causes water loss from water bodies leading to an increase in the concentration of sediments.

In the present study, attempts were made to estimate reference evapotranspiration (ETo) and study its influence on the hydrology of LRC using a physically based model. The ETo values can be considered equal to evaporation from a large body of water, such as a pond or lake. However, for smaller, shallower bodies of water this relationship does not apply. Thus, the ETo is the evapotranspiration rate from a reference surface, not short of water; and is a hypothetical surface with extensive green grass cover with specific characteristics (FAO⁸ ). It expresses the evaporating power of the atmosphere at a specific location and time of the year and does not consider the crop characteristics and soil factors. The ETo plays a key role in irrigation systems design, water management under irrigated and rainfed production (FAO⁸ ). The ETo was used to study the evaporative demand of the atmosphere independently of crop type, crop development and management practices. The site specific water requirement can be estimated, if climatic data recorded at weather station are available in different sites of a region. The ETo component takes a time series input of meteorological data that include radiation, temperature, vapour pressure deficit, and the rate of increase of saturation vapour pressure with temperature.

With improvements in computer technology and simulation techniques, mathematical models are now widely used to predict the variations in ET in the absence of measurements, given meteorological data describing variations in the climate, and land use data describing variations in the vegetation cover. The ET estimate is now frequently obtained using simulation models the Food and Agricultural Organization (FAO) based Penman Monteith type estimates (Mcglinchey & Inman-Bamber⁹ ). Satellite sensors are also being used in catchment hydrology to estimate and describe the dynamics of ET. A recent study by Jovanovic et al.,² ) used satellite-derived data to estimate and describe the dynamics of ET in South Africa. In the LRC, estimation of ET with computer technology and simulation techniques has been rare. However, the study’s ability to use a physically based model to estimate ET offered the opportunity to understand how ET varies across space and time in the catchment. Time series analysis was also used to estimate variation of ET, surface runoff and stream flow. The only process-based model that is widely used, and that accounts for the influence of vegetation on the ET regime, is the Penman-Monteith formula (Monteith¹⁰ ), which is widely used to compute ETo.

Allen et al.¹¹ proposed the CROPWAT 8.0 to calculate ETo by the Penman-Monteith method. CROPWAT is a computer model for the calculation of crop water requirements and irrigation requirements from existing or new climatic and crop data. It has an option to calculate ETo from these data. The Penman-Monteith equation assumes the reference crop evapotranspiration as that from a hypothetical crop with an assumed height of 0.12m, having a surface resistance of 70 s/m and an albedo of 0.23, closely resembling the surface of green grass of uniform height, actively growing and adequately watered (Allen et al.¹¹ ). In the present study evapotranspiration was determined as daily values that represented the removal of water from the soil by evaporation and plant transpiration using CROPWAT 8.0 software developed by the Food and Agriculture Organization (FAO).

2 THE STUDY AREA

The LRC shown in Figure 1 is located in Vhembe District of the Limpopo Province in northeastern South Africa, between latitudes 22°17'34''S and 23°17'57''S and longitudes 29°49'46''E and 31°23'32''E, and covers an area of about 5941 km² (DWAS⁷ ). The catchment originates from Soutpansberg Mountains and drains into the Limpopo River, which extends into Mozambique, and is generally semi-arid and becomes arid as it progresses eastward. The climate of the area is largely influenced by the Intertropical Convergence Zone (ITCZ), modified by local orographic effects. The region has two seasons; a rainy season from October to April, and a dry season (long) from May to September. Rainfall distribution in the catchment is classified as unimodal, having a rainy season predominantly between the months of October to January with the average annual rainfall of about 200-400 mm (DWAS⁷ ). The mean annual temperature ranges from 18°C to 40°C with high variability. Maximum temperatures are experienced in January and minimum temperatures occur on average in winter. Average wind speed in the catchment is approximately 11km/h with high-speed winds occurring occasionally with long intervals.

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Figure 1: The study area

Topographically, the catchment consists of a relatively rolling landscape, which gives rise to shallow storage dams, which have large water surfaces exposed to evaporation. The topographic feature that characterizes the study area is the Soutpansberg mountain range in the east of the catchment, which reaches an elevation of 1,700m above mean sea level before dropping off into the Limpopo River Valley. The general terrain of mean ridge height approximately 800-1200m is common in some places while in some places the peak reached above 1500m. The predominant soils in upland areas are Leptosols while the lowlands are dominated by Vertisol and Acricsols (FAO⁸ ). Land use practises largely vary from plantation forests interspersed with large scale macadamia and banana plantations in the headwaters interspersed with small farm holdings for subsistence agriculture. Land cover degradation in the catchment is a major cause of environmental concern. The headwaters are gradually getting depleted due to uncontrolled anthropogenic activities by the rising human population (DWAS⁷ ).

3 MATERIAL AND METHODS

3.1 Hydrometeorological Data

The daily hydrological and meteorological data (1960-2014) used were all obtained from public sources-the Department of Water Affairs and Sanitation (DWAS), South African Weather Services (SAWS), and the Agricultural Research Council (ARC). The data include daily data on rainfall from eight rain gauges; stream flow and surface runoff from eight flow gauging stations; and ETo climate data obtained from six weather stations as shown in Figures 2 to 4. ETo climate data include minimum and maximum temperature, wind speed, relative humidity, sunshine hours and solar radiation. The data were subjected to quality check for missing data, consistency and stationarity.

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Figure 2: Rain gauges

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Figure 3: Weather stations for ETo analysis

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Figure 4: Flow gauging stations

3.2 Evapotranspiration Analysis

Daily ETo data were calculated by the Penman-Monteith method using full set of climatic data, consisting of minimum temperature, maximum temperature, wind speed, relative humidity, solar radiation, and sunshine hours of daily records of the observatory as input to CROPWAT 8.0.

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