estimation of the irrigation water requirements of crops … 5.4... · 2019-10-23 · irrigation...

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ESTIMATION OF THE IRRIGATION WATER REQUIREMENTS OF

CROPS PRODUCED ON THE FARM VOLSPRUIT

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ESTIMATION OF THE IRRIGATION WATER REQUIREMENTS OF CROPS

PRODUCED ON THE FARM VOLSPRUIT

Introduction

The purpose of this study was to estimate the water requirements of a range of

crops produced under centre pivot irrigation on the farm Volspruit near

Mokopane, Limpopo Province. Seed maize, sugar beans and wheat (or oats)

are produced in rotation under centre picot irrigation. There is a total of 18

centre pivots in the study area, covering a total area of about 370 ha.

Since it is impossible to establish the exact water volumes used to irrigate

crops without access to measurements (e.g. water meter data), crop water use

and irrigation requirements were estimated with the help of a simulation model.

Soil Water Balance (SWB) model simulations were carried out for the period

January 2006 to December 2008 and January 2011 to June 2012.

Methodology

Study area

The boundaries of the study area and positions of the centre pivot irrigation

systems are indicated in Figure 1. A continuous crop rotation system of maize

in summer, followed by sugar beans (late summer / autumn) and wheat or oats

in winter under irrigation is practiced on the farms in the study area. The types

of crops and approximate planting dates used in the simulations are presented

in Table 1.

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Figure 1: Boundaries of the study area (demarcated by red line) and position of fields under centre pivot irrigation (green circles)

Table 1: The crop types and approximate planting date of each �

Growing period Crop Planting time Summer crop Seed Maize September In-between crop Sugar beans January Winters crop Wheat / oats April / May �

4

Description of the SWB model

The SWB model is a daily time step, generic crop, irrigation scheduling model

that simulates the soil water balance and crop growth from specific crop growth

parameters (Annandale et al., 1996; Barnard et al., 1998; Annandale et al.,

1999). SWB includes a database of parameters for about 130 crops such as

field crops, vegetables, pastures and fruit crops, which makes it suitable to

simulate different cropping system scenarios (Jovanovic & Annandale, 1999).

Several previous studies have shown that this model can simulate soil water

balance components and crop water requirements with acceptable accuracy

(Jovanovic & Annandale, 2000; Annandale et al 2002; Beletse et al 2008).

The SWB model calculates grass reference evapotranspiration (ETo) using the

revised FAO Penman-Monteith methodology (Smith et al., 1996) and then

calculates crop water use from available soil water, ETo and canopy cover.

The following input parameters are required to run the model: planting date,

latitude, altitude, rainfall amounts, as well as daily weather data variables

(maximum and minimum air temperature, maximum and minimum relative

humidity, average wind speed, total solar radiation) (Jovanovic & Annandale,

1999). Daily weather data sets were obtained from ARC-ISCW for three

weather stations in the area, namely Bufland, Roedtan and Zebediela.

Although slight differences in daily weather data occurred between the three

weather stations, average ETo values were comparable. Data for the Bufland

station (located at 24.42745oS and 28.59419oE) was the most complete and

therefore used to run the model.

In addition, soil water content (SWC) at field capacity (FC) and permanent

wilting point (PWP) as well as initial SWC are required for each soil layer.

Volumetric soil water contents at FC and PWP were calculated from the soil

texture, using the empirical functions recommended by Bennie et al. (1988).

5

Modelling procedures

According to the soil survey conducted earlier the pivots are mostly located on

sand to sandy loam Hutton soils of high potential under irrigation. Typical soil

physical characteristics for sandy loam soils were therefore used in the model.

Actual crop water use is more dependent on prevailing weather conditions,

crop stage and irrigation management than soil texture, except for extremely

sandy or clayey soils, which is not the case for the study area.

The model was set to trigger irrigation whenever 20 mm of water was depleted

from the root zone. Irrigation requirement per week was defined as the weekly

crop demand after subtraction of rainfall.

For the simulations it was assumed that a new crop was planted immediately

after the previous crop is harvested. For example, sugar beans is planted early

in January and harvested at around the end of April. Wheat (or oats) is planted

immediately thereafter (in May/June) and harvested in Sept/Oct, whereafter

maize is planted. As a result, the planting dates used for simulations differ

slightly from year to year for the same crop. The SWB model database do not

currently have crop parameters for oats and therefore only wheat was

simulated as winter crop. For the purpose of this investigation it was assumed

that the irrigation requirements of oats would be similar to that of wheat, since

they are both small grain crops.

6

Results

Simulated daily irrigation requirements of the different crops were summed

over weekly periods and are given in Tables 4 to 8. The seasonal rainfall and

irrigation requirements per crop are summarised in Tables 2 and 3.

According to Tables 2 and 3 the total annual irrigation requirements for all

crops do not vary much across years and range from 1223 to 1415 mm per

year. However, there is a substantial variation in irrigation requirements per

crop over the years.

When the weekly irrigation requirements are investigated (Tables 4 – 8), it is

clear that the variation is even more pronounced than the seasonal

requirements. The actual irrigation requirement in any week depends on the

weather conditions experienced during that week, i.e. what was the

evaporative demand and especially, did any rainfall occur? Furthermore, the

20 mm deficit threshold set for the onset of irrigation may influence weekly

irrigation demands (i.e. whenever 20 mm water is depleted from the soil, the

model assumes the farmer will irrigate to refill the profile). Depending on the

amount and distribution of rainfall, irrigation may be skipped in some weeks,

while other weeks may require 2-3 irrigation events of about 20 mm each.

The very tight rotation followed for the simulations implies that the land was

occupied for almost every day of the year, which is probably not possible in

practice. This could impact on the actual weekly water abstraction figures.

7

Table 2: Total rainfall and irrigation requirements per cropping season and per year (2006 – 2008)

Crop Growing Season Total

rainfall (mm)

Total irrigation requirement

(mm)


(m3/ha)

Sugar beans Jan - Apr 2006 376 240 2400

Wheat May - Oct 2006 1 597 5970

Maize Oct 06 - Jan 07 248 386 3860

Total for 2006 624 1223 12230

Sugar beans Jan - Apr 2007 13.3 546 5460

Wheat Apr - Sep 2007 38 563 5630

Maize Oct 07 - Jan 08 451 306 3060

Total for 2007 503 1415 14150

Sugar beans Jan - May 2008 354 304 3040

Wheat May - Oct 2008 37 558 5580

Maize Oct 06 - Feb 09 279 522 5220

Total for 2008 671 1384 13840

Table 3: Total rainfall and irrigation requirements per cropping season and per year (Jan 2011 – Jun 2012)

Crop Growing Season Total

rainfall (mm)


(mm)


(m3/ha)

Sugar beans Jan - Apr 2011 262 402 4020

Wheat May - Oct 2011 63 631 6310

Maize Oct 11 - Feb 12 372 381 3810

Total for 2011 697 1414 14140

Sugar beans Feb - May 2012 59 416 4160

Wheat - - - -

Maize - - - -

Total for 2012 59 416 4160

Table 4: Weekly rainfall and modelled irrigation requirements for 2006

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18

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estimation of the irrigation water requirements of crops … 5.4... · 2019-10-23 · irrigation...

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