SAPWAT
SAPWAT4 arrives
by Pieter van Heerden
Introduction
Earth has so much water , covering two-thirds of its surface area , giving it a blue appearance when seen from outer space . Yet , only a very small portion ( 0.00018 %) of this vast quantity of water is fresh and is found in rivers and streams and can therefore be readily used by man for his requirements . This water must satisfy most of man ’ s personal needs : for producing food and fibre ( FAO , 2002 ); industrial production and for maintaining the environment ( Wikipedia , 2012a ). Water , through its scarcity , especially in water stressed countries , has the potential to become a reason for future conflict . This potential problem is aggravated by the world-wide exponential increase in human population and the resultant ever increasing pressure on fresh water resources ( Alois , 2007 ).
South Africa , with its relatively dry climate , reflects similar water situations to that of many countries world-wide where arid and semi-arid climates dominate the landscape . Adequate food , fodder and fibre production is not possible without irrigated agriculture and where fresh water resources are limited ; the effective use of irrigation water becomes much more important . Basson and Van Niekerk ( 1997 ) reported on the water balances of South Africa , comparing 1996 values with estimates for 2030 . In 1996 , seven of the 19 major drainage basins were over-utilised and it is expected that this will increase to eight by 2030 . Ground water failure commonly occurs in some of the denser populated areas as experienced in the Limpopo and Mpumalanga provinces because of the over-use of ground water resources , similar to the situation found internationally where overuse occurs ( Basson and Van Niekerk , 1997 ; GSSA , 2014 ).
Due to the volumes of irrigation water required for crop production , any improvement in irrigation water management and application efficiency could lead to a reduction in the overall water requirement . This , in turn , could have a large influence on water availability and could delay the expected time a country would “ run out of water ”. Good irrigation water management implies good planning of irrigation water requirements , which , if correctly done , could lead to properly designed irrigation and irrigation water conveyance systems . Furthermore , the planning of allocation of fresh water resources for urban , commercial and industrial needs , as well as mining and irrigated agriculture could be improved if the irrigation water requirement can be estimated with a high degree of accuracy .
The methods of estimating irrigation water requirements for planning purposes have developed over time from values based on observation and experience to sophisticated approaches that use weather data and link that to a crop ’ s growth and development . SAPWAT4 utilises the FAO four stage crop development curve procedure based on relating crop evapotranspiration in each stage to the short grass reference evapotranspiration ( Penman- Monteith approach ) by applying a crop coefficient ( Figure 1 ) linked to climate ( Figure 2 ). The crop coefficient data is based on Allen , et al . ( 1998 ) and Steduto , et al . ( 2012 ) and were further developed according to rules derived with the
help of crop scientists . Editing by the user has been simplified by the provision of options available on drop-down menus . It is envisaged that users concerned with groups of irrigators would develop their own sets of defaults tailored to their conditions .
Figure 1 The FAO four stage crop coefficient curve for linking crop growth to climate
Figure 2 The crop data screen
However , the more sophisticated the approaches have become , the more complicated the calculations and the more variables needed to be considered ( Figure 3 ). SAPWAT4 is an improved version of SAPWAT3 , the program that is extensively applied in South Africa and internationally and was developed to establish a decision-making procedure
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SABI | FEBRUARY / MARCH 2017