SAPWAT
The crop requirement input screen has been redesigned somewhat ( Figure 13 ), while the results page has undergone some changes ( Figure 14 ).
Figure 13 Page 1 of the crop setup screen for estimating irrigation wa-ter requirements
Figure 14 Irrigation requirement estimation results screen
The most notable is the right side of this screen where the layout of the water use efficiency results are shown . These results now include worst case scenario recommendations for design application and cycle lengths . Determination of risk is builtin , with irrigation requirement estimates for different levels of non-exceedance , a handy output which allows the designer and user to design for specific risk levels . SAPWAT4 has the built-in facility to export irrigation requirement data on crop , field or farm , or on higher administrative levels to a variety of spread-sheet and similar facilities . Irrigation requirement output is provided in both millimetres and cubic metres for the irrigated area .
Added functionality
Adapting crop growth to temperature
SAPWAT4 utilises the FAO fourstage crop development curve procedure for relating crop evapotranspiration in each growth stage to short grass based reference evapotranspiration ( Penman-Monteith approach ) by applying a crop coefficient . Typical values of expected average crop coefficients under a mild , standard climatic condition are published in FAO 56 and is applied in SAPWAT4 . However ,
FAO 56 makes no provision for the effect of climate , planting date , management strategies or crop varieties on the individual crop growth and development .
Figure 15 Average temperatures for the five climate regions found in Southern Africa SAPWAT4 provides for this with default stage length values for each of the crops listed for each of the five climatic zones found in Southern Africa and , in addition , has options for each crop where there are differing cultivars and modifies the stage lengths where these are influenced by planting dates and climate . Unfortunately , this is a weak point in SAPWAT4 and other similar programs , such as SAPWAT3 ( Van Heerden , et al ., 2008 ) and CROPWAT ( Smith , 1992 ) because of , usually , a lack of enough data to include such variation in crop growth . One solution is to adapt the growth of crops to thermal time ( Parthasarathi et al ., 2013 ), and this facility is now included in SAPWAT4 .
This module needs further development . At present it can only by used reliably for crops that do not need vernalisation , are not influenced by photoperiodism and are not grown under stress situations .
Figure 16 The SAPWAT4 heat unit calculator for adapting crop growth and development to thermal time
Verifying crop growth and development with measured water use data
A second functionality for correcting crop growth coefficients , is a built-in module that uses measured crop water use data , such as determined with
SABI | FEBRUARY / MARCH 2017 37 a lysimeter , to adapt crop growth coefficients as required ( Allen , et al ., 1998 ). Before and after examples are shown below . This is not a fully automatic process , interaction between the user and program remains active and a number of repeats are usually required to reach an optimum level of fit . The solid lines are the existing crop coefficient curve and the dotted lines show the theoretical ideal based on measured data to which the actual crop growth curve need to be fitted ( Figure 17 to Figure 22 for wheat , peas and maize ).
Figure 17 The SAPWAT4 table Kcb data curve , the observed data and the first repeat proposed Kcb growth curve based on observed data of lysimeter measured ETc data for wheat planted on 3 July 2003 at Kenilworth near Bloemfontein
Purification of water from sources namely :
Rivers Dams Boreholes Municipal Outlets
Water treated for the following uses :
Agricultural Potable water Irrigation
Industrial De-mineralized water Process feed water Waste water recycling
Figure 18 The SAPWAT4 table Kcb data curve , the observed data and the fourth repeat proposed Kcb growth curve based on observed data of lysimeter measured ETc data for wheat planted on 3 July 2003 at Kenilworth near Bloemfontein
Figure 19 The SAPWAT4 table Kcb data curve , the observed data and the first repeat proposed Kcb growth curve based on observed data of lysimeter measured ETc data for peas planted on 27 June 2001 at Kenilworth near Bloemfontein
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Application specific solutions to various problems such as :
Iron & Manganese ( Fe & Mn ) Bacteria & Viruses Low Alkalinity ( Aggressive water ) Brackish water ( NaCl — Salt ) Odour & Taste
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