430 SAJEMS NS Vol 5 (2002) No 2 The Leaching Paradox and Return Flow Management Options for Sustainable Irrigated Agriculture R J Armour and M F Vlljoeo Department of Agricultural &onomics, University of the Free State ABSTRACT Leaching is necessary to maintain an acceptable salt balance in the root-zone of irrigated crops. This however contributes to point and non-point source water pollution externalities if not managed correctly. The use of a linear programming model, SALMOD (Salinity and Leaching Model for Optimal Irrigation Development) is demonstrated to determine the feasibility of leaching. artificial drainage, and on-farm storage/evaporation ponds to manage degraded return flows entering the water source and groundwater. Results show optimal cropping compositions and management practices to maximise farm returns subject to water quality conditions and return flow constraints. The economic effects of constraining return-flows and of water pricing policy on the volume of return flows are also determined. Results show valuable policy information regarding the interactions between artificial drainage subsidisation, return flow restrictions and on-farm storage. JELQOO,Q25 1 INTRODDCTION With Sub-Saharan Africa having by far the highest population growth rate in the world (2.9 per cent per annum), the imminent threat of HIV/AIDS that's crippling the workforce, weather changes brought about by global climate change and the drastic slump in the regional economy, food shortages in this region loom in the not too distant future. In Sub-Saharan Africa the potential irrigable area is estimated at 33 million hectares. Presently only 13 per cent of this irrigable area is utilised for crop production. With the stability of production and increased yields offered by irrigation, tremendous pressure is going to be placed on expanding the potentially irrigated area in Sub-Saharan Africa. This will be at a disastrous cost to the environment and hence on the sustainability of new and existing schemes if the necessary precautions are not taken. R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 00 9) . SAJEMS NS Vol 5 (2002) No 2 431 In a study by Seckler et al. (1999) titled Water Scarcity in the Twentieth Century, South Africa is classified under category I: "These countries face 'absolute water scarcity'. They will not be able to meet water needs in the year 2025." Water use efficiency in irrigation agriculture will thus also become crucial as per capita demand for water increases (Basson et ai., 1997). "There are ( also) clear indications, ... , that the price of water for all uses including irrigation will be adjusted upwards to better reflect the cost of supply or perhaps even its value" (Backeberg et aI., 1996: 12). Backeberg et al. (1996: 22), further states that "water quality is becoming of increasing concern to irrigation, both from a supply point of view and with respect to the environmental impacts of irrigation." In 1995 in South Africa alone about 110 000 hectares of irrigated land were already affected by waterlogging and/or salinisation. Currently irrigation in Sub-Saharan Africa is by far the largest user of stored water, using 83 per cent, and in South Africa 53 per cent (Backeberg et aI., 1996). With total water demand expected to exceed supply before 2020, industry and urban users are going to be competing strongly for this valuable resource. The price-cost squeeze experienced by farmers over the last few decades, the weakening terms of trade, recent drastic fuel price increases and the increasing cost of labour further jeopardise the economic sustainability of irrigation agriculture, an industry so crucial for the economies of many rural areas. A major factor that could possibly further jeopardise the sustainability of irrigated agriculture, but which can be effectively controlled, is the accumulation of minerals salts in irrigated soils, which results in a breakdown of soil structure and accumulate to levels toxic to the crops grown. According to Gouws et ai. (1998: 8) the three water quality components that have a financial impact on crop production are the ''lotal salt effect, specific ion toxicity and sodium effect on soil properties". The concentration of dissolved salts, be it from natural or anthropogenic causes, currently poses the greatest threat within the study area. "The rise and fall of a number of past civilizations have been linked to their ability to sustain irrigated agriculture. The inability to control salinisation and degradation of irrigated lands are mostly viewed as the main causes for their decline" (DW AF, 1993). No irrigation system is sustainable without sufficient drainage. Unless natural drainage till below the root zone is sufficient and water tables are not rising, artificial drainage has to be installed. According to Du Preez et al. (2000: 154) "Results from these estimations (Szabolcs model) indicate that all the undrained soils will, due to excessive salt accumulation, become unsuitable for irrigation by approximately the year 2050." To reinforce this, Brady and Weil (1996: 307) state that "if the irrigation system does not provide good internal drainage, soil R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 00 9) . 432 SAJEMS NS Vol 5 (2002) No 2 salinity can increase to intolerable levels, as can the exchangeable sodium level. The latter engenders chemical and physical problems that, if not corrected, will render a soil virtually useless as a habitat for plants." Leaching is the process of applying water over and above the requirements of the plants irrigated. It is a management practice used to "flush" a certain amount of accumulated salts out of the root zone to maintain an acceptable salt balance. This practice is often considered by non-specialists as wasteful, especially as irrigation engineers and scientists appear to be in doubt about the required leaching rates and the efficiency of the leaching practice (Kijne et al., 1998). To leach effectively, soils should have a good infiltration rate till beyond the root zone. In heavy soils and where waterlogging occurs, artificial drainage is required. The heavier the soils, the more expensive the costs of installing the artificial drainage. Thus the benefits of leaching need to be quantified to be able to justify the capital expenses involved. Leachate flows back into the river or groundwater carrying high concentrations of salts, further degrading the water source and creating secondary costs through externalities for downstream users. The paradox, however, is that without leaching salts (those inherently found in soil or those deposited by irrigating with poor water quality) out of the soil, salts build up, degrading the soil to levels that can no longer support viable crop production. Improper leachate management results in downstream water degradation, rendering it less suitable for other users, and damaging the environment. It may cause watertables to rise and flushes expensive nitrogen applied to the fields, carrying with it other agriCUltural chemicals. The importance of irrigation has been stressed and leaching is essential for its long-term sustainability. In the past, the government subsidised the installation of artificial drainages. However, subsidising drainage creates an incentive to leach more. The negative externalities created by return- flows thus need to be managed. Using SALMOD, (Salinity and Leaching Model for Optimal Irrigation Development) the economic impact of constraining return-flows is determined. Incorporating into the model an option of building an on-farm storage dam to manage return flows, also makes it possible to determine whether or not it is cost effective to build the dam. The main purpose of SALMOD is to optimise the farm level total gross margin above specified costs (TGMASC) to ensure optimal resource use and to identify the constraints preventing the maximum TGMASC form being attained. This is achieved by calculating with linear programming the optimal crop combination for a farm's specific physical resource endowment, subject to various R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 00 9) . SAJEMS NS Vol 5 (2002) No 2 433 constraints. Particularly useful data generated by the SALMOD, are the dual or shadow values of the constraints. These values indicate how much can be paid for one extra unit of a constraining factor, for example how much can be paid for one extra mmlha on top of the existing quota for inigation water. Figure 1 A schematic representation of the positioning of the OVIB study area within the regionaJ hydrology A SCHEMARIC LAYOUT OFmE HYDRAULIC SYSTEM IMPACI1NG ON THE STIJDY AREA r-----------------------------, 1 AflmooCIDl owg .. WIir I < I 3 THE STUDY AREA ~ j'ViNJ.HOItS ~7 '/(911/011 SdI.,.. I LEGEND River ~ canal ~ 0<1'11 0 Farma- inlef'lleYllld D Irrfgafoo S~ '----.' Slu~hee BOU'lOOIy Douglas, the main town within the study area (see Figure I), is a thriving community based entirely on the forward and backward linkages of the irrigation industry. The initial Bucklands and Atherton inigation plots allocated were part of a government social-economic scheme after the drought and depression of the 1930s (DWAF, 1993: 14). The sustainability of the soils on which these plots were established for inigation agriculture was not a primary factor as they were developed mainly for socio-economic purposes. In 1984 an Irrigation Board was established to manage water allocations in the demarcated area. Currently water is charged for on a per hectare basis and not on a volumetric basis, which distorts incentives for efficiency in inigation water application. R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 00 9) . 434 SAJEMS NS Vol 5 (2002) No 2 Figure 2 Salinity fluctuations measured as EC(mS/m) and TDS(mgll) at the Douglas Barrage on tbe Vaal River, DW AF 1977-1997 VAAL RIVER AT DOUGLAS BARAGE WALL 1800,----------========----------,300 1600 +-----------------------j 250 1400 t--------/----------------j ,200 +--------£'-----------------+200 i 1000+----*---~----+_------~--~ ~ S 1501 ~ 800 +--+--~ hl 400 50 200 0 .... '" '" 0 '" '" ;z ." "" ... a> '" 0 ;;; '" .., ~ "' <0 .... .... "" f- a> '" a> 00 '" '" a> :~:'45 per cent clay). 165 hectares are under a centre pivot irrigation system (CPI) while the remaining 35 hectares are flood irrigated (FIS) and there is no drip irrigation systems (DIS) used. 100 hectares of the irrigable area has sufficient natural drainage (NDS), 70 hectares have limited drainage (LDS), 20 hectares are artificially drained (ADS) and the remaining 10 hectares are waterlogged (WLS). Table 4 Olierivier 1998 monthly average Eciw (mS/m) s I Oct f Nov I Dec I 119 i 130 I 113 i 97 Source: ovm The monthly average electrical conductivity of the irrigation water (ECiw), measured in milli-Siemens per meter (mS/m), is depicted in table 4. The annual average of these monthly average ECiw values measured by ovm through the year in 1998 (OL98) is 98.25 mS/m and is used in Table to set up a range of water qualities incrementally varied at positive and negative intervals of 10 per cent. This range of water qualities is broadened in a forthcoming WRC report on which this paper is based, where SALMOD is run for a range of predicted water qualities as determined by Du Preez et al., 2000: 18. TableS The annual average ECiw varied prametrically from the 1998 ovm reading for Olierivier Mn3 I MD2 MDI ; OL98 PLI PU I PL3 i Parametric range -30% -20010 "TlO% i OL98 +10% ; +20% +30% ""-~ Annual Average l I i i I ECiw (mS/m) 68.8 78.6 ! 88.4 98.3 108.1 117.9 127.7 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 00 9) . 442 SAJEMS NS Vol 5 (2002) No 2 Table 6 shows the change in TGMASC, water fine and return-flows over the parametric range of water quality variations. The dual values are zero because return-flows are not constrained. Table 6 Percentage change in TGMASC (R), total fine (R) & return- flows (mm) from the OVIB 1998 ECiw results for a parametric run with no management options, Olierivier case study farm (2000) Parametric mo.=de=l:...:r-=u=n-=fi.=.:or:....:: __________ --l r---------;MN3 : MN2 . MNl: EC98-=---;,_P_L-'-1_-'-PL_2-=--_P-L-'C3----1 Total Gross i Margin i 2.8% ! 2.1 % ' 1.0% l R 908 278=--;-i _-6~-,.-,,-6°,-,%,-,-----=1-:,-4.=2-,-,%,-,-, _--=2-,-7.=2-,-,%'-1 Total Water Fine i O.O%! 0.0%' 0.0% i R 3S 673 i 0.0%. 0.0%: 0.0% Return-flows i -3.4%1 10.1%: 10.1%, 13 408 mml 13.0% 167.8%; 173.7% Dual i 0%1 O%! 0%, 0 i 0%; 0% Table 7 shows the change in optimal crop composition over ECiw varied parametrically. Wheat replaces maize at EC98 but, interestingly enough, at EC98 + 20% (PL2) maize replaces lucerne resulting in a 167.8 per cent increase in return-flows (Table 6) from the EC98 level. Using the Olierivier case study farmer's own CEBs, SALMOn is set up to choose only between wheat, maize, groundnuts, potato and lucerne. Table 7 Wheat Maize Optimal crop composition (hectares) for a parametric run with no management options using OVIB 1998 ECiw values as basis, Olierivier case-study farm (2000) Optimal crop composition : MN3 I MN2 i MNI ! EC98 ~ PLI PL2 PL3 I ; i 40.0 I 43.8 58.2 30.0 ! i 43.0 46.2 46.2 i 1.0 , 21.0 1 ; : Groundnut I I Potato : 6.0 I I 6.0 6.0 , 6.0 6.0 6.0 6.0 Lucerne i 141.0 I 137.8 : 137.8 I 143.0 i 140.2 : 104.8 . 130.4 In Table 8 it can clearly be seen how the productive value of irrigation water decreases as the water quality deteriorates. In all water after-year fme rows (WFI-4) the shadow price decreases from left to right. For the pre-year water fine row (WFPY) this is also true except for column PL2 where maize is brought into the optimal crop composition again placing a higher potential value on pre-year water resulting in the deviation at PL2. In Table 8 the dual prices R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 00 9) . SAJEMS NS Vol 5 (2002) No 2 443 for extra irrigation water in both the pre- and after-years change significantly as water quality deteriorates. Table 8 Change in water fine shadow values (R) from the OVIB 1998 ECiw results for a parametric run with no management options, Olierivier case-study farm, 2000 Water fine shadow values MN3 , MN2 MNI I EC98 PLI PL2 , PL3 , I WFPY 1.83 ; 1.66 I 1.69 , 1.70 0.94 ! 1.58 0.47 ! WFI 2.58 2.41 ; 2.43 2.45 1.69 1.66 1.21 ! , , WF2 , 2.49 ! 2.32 2.35 j 2.36 ; 1.60 [ 1.57 1.13 WF3 2.41 i 2.24 2.26 , 2.28 i 1.52 I 1.49 ! 1.04 WF4 2.32 2.15 2.18 2.19 i 1.43 : 1.40 i 0.96 , , Figure 3 TGMASC for the Olierivier case-study farm using OVIB 1998 ECiw readings varied parametrically, with and without return- flows constrained and fixed capital management options implemented, 2000 TG~SC for Q!!erlYler case-stu~ f!!m usl!:!g OVIB , 11118 !l;CIW varied uarametr1c!lIX I-+- 01. ...... Ol~c .:r- OLrrno --- OLnmoRft I 950 900 ....... ~ 850 ~ 1800 u ~~ i 750 ~~ ~ E 700 "'~ • ... 650 ~ 600 i< 550 MN3 MN2 MN1 EC98 PL1 PL2 PL3 1998 ECiwplul (PL) &minul (MN) 10, 20 &30% Figure 3 shows the maximum attainable TGMASC for the Olierivier case-study farm at the 1998 ECiw varied parametrically for various scenarios, If the irrigation water quality were to be improved by 10 to 30 per cent from the 1998 I i I I ! i R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 00 9) . 444 SAJEMS NS Vol 5 (2002) No 2 ECiw average, constraining return-flows would have no effect as can be seen by the OL and Olrfc, and also the OLnmo and OLnmoRfc lines coinciding over MN3, MN2, MN1 and EC98. What this shows, is that if policy is implemented to constrain return-flows, water quality will be improved and prevented from deteriorating further. Under these improved water quality conditions, the return- flows from the resulting optimal crop composition will be less than the maximum specified in the constraint, making the return-flows constraint no longer necessary once farmers are using and managing their on- farm storage dams properly. However, farmers have to be convinced to install drainage and to build on-farm storage dams. If water quality were to deteriorate form PLl to PL3, TGMASC decreases substantially as can also be seen in Table 9. For a water quality deterioration of 30 per cent, Table 9 shows a 35 per cent reduction from the attainable TGMASC modelled under 1998 ECiw conditions when no management options are implemented and return-flows are constrained (row OLnmoRfc and column PL3). A 25 per cent reduction in TGMASC is attainable at the same water quality conditions if return-flows are not constrained and management options implemented (row OL and column PL3). The impact of constraining irrigation return-flows only starts to have an effect once water quality deteriorates till below 1998 ECiw levels. Table 9 TGMASC (RIfarm) for parametrically changed 1998 ECiw values for the Olierivier case-study farmer, 2000 MN3 MN2 MNI EC98 PLl PL2 PL3 _ ..... Ave.Annual ECiw (~S/m) _68.6 78.4 88.2 98 107.8 117.6 127.4 OL 4.0% 3.5% 2.4% 1.4% -4.8% -12.9% -25.3% QJrfc 4.0% 3.5% 2.4% 1.4% -5.4% -15.5% -33.4% Olnmo 2.8% 2.1% 1.0% R908278 -6.6% -14.2% -27.2% -c:::. 2.8% OlnmoRfc 2.0010 1.0% 0.0% -7.1% -17.2% -35.2% Table 10 The impact of fixed capital management options on artificial drainage installation (ha's) brought into the optimal solution for the Olierivier case-study farm using 1998 OVID ECiw values, 2000 Soil Trans.WL-AD: LMS : SNL , SNC i CLY FIS : 0 5 • ____ .9 , 0 r CPI 0 5 ! 0 i 0 i , R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 00 9) . SAJEMS NS Vol 5 (2002) No 2 445 The management options determined by SALMOD to realise the optimal TGMASC for the 1998 ECiw scenario are shown in Table 10 SALMOD calculates that installing artificial drainage to convert 10 hectares of waterlogged sandy-loam soils, 5 of which are flood irrigated and 5 hectares under centre pivot, to fully artificially drained soils will bring about a 1.4 per cent (see Table 9, OL-98) increase in TGMASC after the annualised costs of this option are deducted. Table 11 Water over use volumes, fines (Cost) and shadow price (Dual) results for the Olierivier case-study farm using 1998 OVIB ECiw data, 2000 Stepped tariff i _Y!'Jume {mm} Cost {R} i Dual(R) WFI I 14100 3596 2.4473'- WF2 1 14100 -t-- 4794 I 2.3623 --==-=---~-- : WF3 i 14100 5993 I 2.2773 WF4 i 14100 ; 7191 , 2.1923 WFPY I 14100 14100 , 1.7023 Table 11 indicates that the volume of the irrigation quota is most constraining. At the current water price and stepped water overuse fme structure, all 4 levels of the after-year fine (WF1-4) and the full pre-year fme (WFPY) volumes are fully utilised. This is true for all incremental water quality scenarios that the model was run at for Olierivier. This is partially because more inigable land is available (200 ha's) than water rights (141 ha's) to inigate all the land with. The dual of the first after-year fine tier (R2,45) indicates that for every 1 extra millimetre per hectare of water rights available at that specific charge rate (RO.17 + RO.17 x 50 per cent per mmlha), an extra R2,45 /ha would be added to the TGMASC. This indicates that for every 26.5 cents that the farmer currently pays for the 1st tier of water overuse, he makes 244.7 cents, and thus indirectly could afford to pay up to 244.7 cents per millimetre per hectare for that water. As water quality however changes (see Table 8) the dual prices for irrigation water change quite markedly. The impact of changing the price of irrigation water for Olierivier Table 12 shows the change in the water fme rates as the water price is increased from RO.17 per mmlha to R1.70 per mmlha. Water fines for overuse in the after-year are directly linked to the water price while in the dry pre-year the water overuse fine is fixed at R1.00 with the ovm 2000 water pricing structure. The analysis in Table 13 based on this range of prices, includes the R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 00 9) . 446 SAJEMS NS Vol 5 (2002) No 2 water fine for the pre-year (WFPY) at the fixed value of Rl.00 per mmlha for all water prices. The shadow value of WFPY at the increased rate of R0.34 per mmIha is R1.92 indicating that it would not be feasible to use extra water in the pre-year at R2.00 per mmlha as for each R2.00 spent for each I mmlha extra pre-year water TGMASC will only increase by RI.92. Table 13 The water fine tariff structure for the OVIB in response to increases in the price of water : Water price(R!mmlha) Table 13 shows the impact of increasing the price of inigation water on TGMASC, water fine costs, return-flows, the optimal crop composition and the shadow prices of the water fines as the water price is increased from RO.17 per mmlha to RI.70 per mmlha. In Table 13 we see that at the full volume of pre-year extra water allowed, subject to the pre-year water fine (WFPy), remains fully utilised as the water price is increased (indicated by positive shadow values) because the pre-year water fine is not linked to the water price, as are the after-year stepped fmes. Negative after-year water fine shadow values show the decrease in the fine water price needed before that tier of water can be used profitably on the farm. As water quota costs and water overuse fine costs are included as production costs in SALMOn, it was found in the farm-level results (not shown in this report) that the increasing cost of water causes production capital to become constraining. Increasing the price of irrigation water results in less return-flows, but only after a 6-fold increase in the cost of irrigation water, at which rate all the extra water is no longer viable to use to leach. Increasing the price of irrigation water is therefore not a sustainable irrigation policy to reduce the agricultural retum- flows as it provides an incentive not to leach that will lead to the building up of salts in the vadose zone. R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 00 9) . SAJEMS NS Vol 5 (2002) No 2 447 Table 13 The impact of a change in irrigation water prices on TGMASC, total excess water use fine, return-flows, crop composition and water fine shadow values for 1998 ovm ECiw data for the Olierivier case-study farm, 2000 Water price. ' I . l-~mlhaL_· ~-J 0.34 : 0.68 · 1.~1.36 1.70 Total Gross MarginJ~L 964 272 . 916557 . 815092 727246' 671 133 :630436 Total Water Fine (R) • 35673 : 57246 ; 100392 : 64 437 14100 i 14100 ~~-!lowti...Il!!!0!~!L 139 11 139 11 139 , 9729 : 8319 8319 Optimal crop composition (ha) ~---~~----~-1 I 25.42 '13.29 0 0 0 0 Wheat !-'M=a=iz=e'-------________ :----=9c..::1-=.5---'-4_i, 77.49 34.88 52.59, 87.85 ! 65.34 Groundnut 0 ~ , 0 0' 0 ' 0 Potato 6.00 : 6.00 6.00 6.00! 6.00 ! 6.00 Cotton o 0 0 0 0 0 ~e~~--__ --~~10~2~.4~6~'~I~I~6.~5~1~!~14~6~.9~~1~16~.0~5~_7~5~.5~1'_______,~8~7-=.9~2~ Water fine shadow values (r) rwFPY-----·--·------~----::-2--::.0--:-3 -------,'--1,----,.9:'-:c2--'-----';~'1-. 7""8--'--=----;i~I-.5-4-,----1.3-::-0,-------I-.0'--:i6----1 WFI 2.85 I 2.47 ! 1.75 • 0.87 I -0.02 I -0.90 WF2 i , 2.76 I 2.28 . 1.32 0.22 i -0.88 i -1.97 WF3 2.67 I 2.09 ! 0.89 -0.42·---r-------'--1-"-.7-'----3 -: --:i_3--":.0---=-5--l WF4 I 2.57 1.90 0.47 -1.06 -2.59 -4.12 CONCLUSIONS What the automatic leaching fraction and yield percentage management option results show are that at current water prices, the economic impact of accepting a reduction in yield is greater than the cost of applying extra water to leach accumulated salts from the soil to attain a better yield. At current water prices SALMOD results indicate that the maximum yield is selected with as much leaching as required subject to the drainage status constraint of the specific soil. The results clearly show that the benefits fonn leaching more as water quality deteriorates, to obtain a 100 per cent yield, outweigh the costs of leaching until return-flows become constraining. It is also clear fonn the results that where irrigation rights exceed irrigable area, irrigation water quantity is generally sufficient and the shadow prices of water overuse fines are generally lower than where irrigable area far exceeds irrigation rights. Furthennore, even with the high electricity costs of pumping irrigation water, SALMOD results show that the productive value of the extra water far exceeds the stepped fmes charged for exceeding water quota allocations. R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 00 9) . 448 SAJEMS NS Vol 5 (2002) No 2 When conducting the farm level survey, the impression gained was that where the irrigable area far exceeded the irrigation quota, it was a cheaper alternative to move the irrigation system to new land than to remediate old land. Jrrigable land without water rights can be purchased for R7000 per hectare while the cost of installing artificial drainage could exceed R 15000 per hectare. The purchase of additional land was however not. an option included in the model. This practice is however unsustainable and very environmentally unfriendly. The subsidisation of the costs of artificial drainage on farms (implemented in SALMOD by leaving the costs of drainage installation out of the objective function and production capital constraints), results in an increase in the volumes of return flows when return-flow volumes aren't constrained, which could actually further exacerbate the water quality problem. Subsidising irrigation drainage thus has to be implemented together with return-flow constraining/effective management policy. By implementing policy constraining return-flows, water quality will be improved and prevented from deteriorating further. Under these improved water qualities the return-flows of the resulting optimal crop composition will be less than the maximum specified in the constraint, making the return-flows constraint no longer necessary once farmers are using and managing their on- fann storage darns properly, but are initially required to get farmers to install drainage and build on-farm storage dams. The scenario runs also show that when production capital is constraining or limited, the capital will rather be used for production inputs than for implementing long-tenn capital improvements. Maize and potato have the same sensitivity and gradient and are the most sensitive crops to salinity of the 6 included in SALMOD. Potatoes being by far the highest value crop are included for all water quality situations and take up the ideal soils leaving little room for maize. Maize requires large leaching fractions, so if water quota is constraining and well-leached soil is still available after potatoes have been included, other crops are brought into the optimal crop composition instead of maize. The shadow prices of the stepped water overuse fmes indicate how much a farmer could pay for water if a water market in which water rights could be freely traded existed. For the Olierivier farmer for example, only at eight times the current price of irrigation water is it no longer feasible to use extra irrigation water at a stepped fine rate. R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 00 9) . SAJEMS NS Vol 5 (2002) No 2 449 FURTHER RESEARCH NEEDS By understanding the full dynamics and interactions between irrigation water quality and the soil salinity status on crop yield over irrigated time, mistakes made in the past by choosing unsustainable irrigation sites can be prevented. Furthermore the impact of various natural or artificial (e.g. policy mechanism) scenarios on existing schemes could be more accurately modelled, leading to increased economic efficiency and sustainability of the irrigation industry as a whole. However, current USDA Salinity Laboratory evidence suggests these interactions are far more complex than originally thought. .... Rhoades, the doyen of soil/plant/salinity interactions, contends that no one has succeeded in combining all the refinements necessary to overcome the inherent problems of relatively simple salt balance models and geophysical sensors, to address the enormous field variability of infiltration and leaching rates (Blackwell et a1., 2000). Current literature and research on salinity management in irrigation agriculture also fails to capture the stochastic nature of inter-seasonal irrigation water quality as well as the cumulative economic and sustainability effects of irrigating with stochastic water quality levels. Further limitations for setting criteria for salinity include: (i) the need to make assumptions about the relationship between soil saturation extract salinity (for which yield response data is available) and soil solution salinity. (ii) the deviation of the salinity of the soil saturation extract from the mean soil profile salinity, to which crops would respond. (iii) The criteria for crop salt tolerance do not consider differences in crop tolerance during different growth stages (DW AF, ]996). ENDNOTE This paper is based on preliminary fmdings of a current Water Research Commission (WRC) project due for completion by the end of 200 I. Financial support from the WRC to enable this research, and from USAID to attend this conference is hereby acknowledged R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 00 9) . 450 SAJEMS NS Vol 5 (2002) No 2 REFERENCES ARMOUR, R.J. & VILJOEN, M.F. (forthcoming) "The Economic Impact of Changing Water Quality on Irrigated Agriculture in the Lower Vaal and Riet Rivers", WRC Report No. 94711/01, Pretoria. 2 ASCE (1990) Agricultural Salinity Assessment and Management, Manuals and Reports on Engineering Practice No. 71. TANn, K.K. (ed.) American Society of Civil Engineers: NY. 3 AYERS, R.S. & WESTCOT, D.W. 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