ESTIMATION OF THE IRRIGATION WATER …

1 ESTIMATION OF THE IRRIGATION WATER requirements OF CROPS PRODUCED ON THE FARM VOLSPRUIT ! " # 2 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 ( WATER meter data), crop WATER use and IRRIGATION requirements were estimated with the help of a simulation model.

2 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. 3 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.)

3 , 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).

4 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 and ) 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 .

5 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.

6 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.

7 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, 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 ( whenever 20 mm WATER is depleted from the soil, the model assumes the farmer will irrigate to refill the profile).

8 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) Total IRRIGATION requirement (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 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.

9 Total rainfall and IRRIGATION requirements per cropping season and per year (Jan 2011 Jun 2012) Crop Growing Season Total rainfall (mm) Total IRRIGATION requirement (mm) Total IRRIGATION requirement (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 $ % & ' ( ) $ * + , $ - . + , % & * + /0 &, % & - . + /0 &, % & - . + 1/2 /0 &, 9 3 # 4 + (, $ % & ' ( ) $ * + , $ - . + , % & * + /0 &, % & - . + /0 &, % & - . + 1/2 /0 &, 10 Table 5: Weekly rainfall and modelled IRRIGATION requirements for 2007 $ % & ' ( ) $ * + , $.)

10 + , % & * + /0 &, % & - . + /0 &, % & - . + 1/2 /0 &, 11 3 # 4 + (, $ % & ' ( ) $ * + , $ - . + , % & * + /0 &, % & - . + /0 &, % & - . + 1/2 /0 &, 12 Table 6: Weekly rainfall and modelled IRRIGATION requirements for 2008 $ % & ' ( ) $ * + , $ - . + , % & * + /0 &, % & - . + /0 &, % & - . + 1/2 /0 &, 13 3 # "4 + (, $ % & ' ( ) $ * + , $ - . + , % & * + /0 &, % & - . + /0 &, % & - . + 1/2 /0 &, 14 Table 7: Weekly rainfall and modelled IRRIGATION requirements for 2011 $ % & ' ( ) $ * + , $ - . + , % & * + /0 &, % & - . + /0 &, % & - . + 1/2 /0 &, 15 3 # 54 + (, $ % & ' ( ) $ * + , $ - . + , % & * + /0 &, % & - . + /0 &, % &.)))