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IMPASSE: A DSS for Assessing Impact of Saline and Non-Saline Irrigation Waters on Crop Yield Reductions

Ravinder Kaur and Rashmi Malik

Division of Environmental Sciences, Indian Agricultural Research Institute, New Delhi-12, India Email: rk_home@yahoo.com

Abstract

A field scale, user-friendly Decision Support System (DSS) named IMPASSE (IMPact Assessment & management of Saline/ Sodic Environments) has been developed for assessing impacts of conjunctive water use practices on soil salinization and crop yield reductions. Such tools provide a time-efficient and cost-effective means for maintaining sustainable agriculture. The DSS has been extensively tested and validated on 11 fields in 6 villages of Gurgaon district and on controlled experimental fields in Sampla experimental station of Karnal district in Haryana (India). The average relative wheat yield reductions due to salinity, under various saline water-irrigation treatments, could be realistically simulated by the DSS. Proposed DSS based long-term (2001-2010) impact assessment of existing (cyclic) and 8-alternate (cyclic and blending) conjunctive water use strategies, in a test farmer’s field, suggested that cyclic mode was superior to the blending mode of irrigation during both Paddy and Wheat growing seasons. It further suggested that a change in the conjunctive water use plan for the Wheat growing season was associated with minimum additional demand for fresh water supplies, reduced root zone salinizations/ sodifications and 3-4% higher long term returns to the test farmer.

Media Summary

A field scale decision support tool for assessing environmental impacts of different conjunctive use strategies and proposing effective conjunctive water use plans for sustained/ improved crop productivities has been developed.

Key Words

EIA, Conjunctive Water Use, Crop Salt Response, Wastewater Irrigation

Introduction

Saline ground waters pose a continuous threat to sustained irrigated agriculture in many arid and semi-arid regions. The efficient substitution of low salinity water for saline water is generally aimed at minimizing yield losses and enhancing flexibility of cropping without much alteration in the farming operations. Strategies available include blending and cyclic use of non-saline/ saline waters, either per irrigation or seasonally. The pros and cons of such options have been discussed at length by Rhoades (1990) and Minhas and Gupta (1992). Indirect evidence seems to favour cyclic use of multi-salinity waters (Minhas and Gupta, 1992; Naresh et al., 1993). However the two strategies have not been widely tested under similar conditions.

Appropriate decisions on the conjunctive use of good and poor quality irrigation waters on chemically degraded lands requires the decision maker to predict the future fate/distribution of soil salinity / sodicity build-up under management regimes suggested by local-resource management plans. The decision makers aim is to maintain sustainable agriculture by proposing efficient use of finite natural resources through say better conjunctive water use plans. One way to address this need is to conduct actual site-specific long-term field experiments. However attempts to extrapolate the information gathered from a few homogenous experimental plots to other homogenous experimental plots or to large heterogeneous areas with widely varying physiographic, climatic and hydrologic conditions suffers from subjectivity and often results in unacceptable errors. Further, environmental impacts of land and water use decisions are often not apparent until long after decisions are implemented. Thus general guidelines or decisions on land and water use are often not feasible. It is in this context that the use of a decision support system such, as the one proposed here, comes to the forefront.

Methods

Proposed DSS Development

The proposed user-friendly DSS, IMPASSE (acronym for IMPact Assessment and management of Saline/ Sodic Environments) comprises of a single layer (root zone) salt-water balance capacity-model. From the knowledge of the daily water inputs and losses, and of soil-solute chemical interactions, IMPASSE predicts average root zone electrical conductivity (EC) and exchangeable sodium percentage (ESP) of the soil saturation extract, at a daily time step. For root zone salinity (EC)/ sodicity (ESP) exceeding the threshold of any given crop, the relative yield reductions due to salinity (RYRs) and sodicity (RYRa) stresses in the proposed DSS, were individually estimated as:

RYRs = Sec * (EC – ECt)
RYRa = Sesp * (ESP – ESPt)
While the combined effects of both salinity and sodicity stresses on the relative crop yield reductions, on a particular day (RYRcdi) and for the whole crop growth period (RYRcg), were assessed as:
RYRcd = MAX (RYRs, RYRa)
RYRcg = MAX (RYRcd1, RYRcd2, RYRcdi)

Proposed DSS Calibration & Validation

The proposed DSS basically requires the determination of the leaching fraction, saturated hydraulic conductivity and maximum allowable moisture deficiency input parameters through calibration. The direct determination of these parameters is generally very cumbersome, error-prone and non-feasible. These calibration parameters were increased or decreased in steps, keeping in view their natural uncertainty under field conditions, till good correlation coefficients between the observed and simulated root zone EC, ESP, Na+, Ca2+ and Mg2+ ion concentrations were obtained. These calibrated parameters were then used for the testing and validation of the proposed DSS. For validating the DSS’s potential to realistically simulate impacts of existing land and water management strategies, 11 farmers fields in 6 villages of Sohna block of Gurgaon district of Haryana state in India (Fig. 1) were selected and a detailed inventory on the weather, farming practices, soils and waters of the study area was prepared for 2000-2003 period. The secondary data from the controlled experimental fields in Sampla experimental station of Karnal district in Haryana was also used for the test/ validation of the proposed DSS.

Proposed DSS Application

Potential of the proposed DSS to assess long-term (2001-2010) impacts of both alternate and existing Paddy (i.e. 3TW, 3CW, 3TW?????) and Wheat crop season (i.e. 2CW, TW, CW?????) conjunctive water use practices on the soil salinization/ sodification and relative crop yield reductions was evaluated through its application on a test-farm in the Khatrika village of Gurgaon district. This representative farm was associated with the highest initial root zone salinity (6.7 dS/m) and poorest quality tube well waters (EC: 4.21-4.30 dS/m; SAR: 6.59-12.74???? and RSC????: 0.00-4.06 meq/l). CAN THESE (????) BE CLARIFIED BY REFERING TO FIGS WHICH SHOULD BE CLEARER Long-term impacts of both blending and cyclic modes of conjunctively applied canal (CW) and tube well (TW) irrigation waters were assessed. The 8-Paddy season alternate conjunctive water use strategies comprised of 9- irrigations with cyclic applications of CW followed by TW (CW: TW); TW followed by CW (TW: CW); 4CW followed by 5TW; CW, 2TW, 3CW, 3TW; 1CW followed by 8TW; 9TW and with 50%CW blended with 50%TW and 25%CW blended with 75%TW. While the 8-Wheat season alternate conjunctive water use strategies comprised of 4- irrigations with cyclic applications of all CW; 1CW, 1TW, 2CW; CW, TW, CW, TW; 2CW, 2TW and 1CW, 3TW and with 25%CW blended with 75%TW; 50%CW blended with 50%TW and 75% CW blended with 25%TW. For simulating the impacts of these conjunctive water use practices it was assumed that the farmer of the representative farm followed a particular land / water management practice for the entire period of the simulation and that the weather during the simulation period was the same as that for the year 2000. This assumption was based on the analysis of long-term monthly weather records, which showed that the weather for the year 2000 was quite close to the test area’s average weather.

Results

The validation of the developed decision support tool (IMPASSE) on both farmer’s fields and controlled experimental fields revealed that the mean relative errors associated with the soil salinity/sodicity predictions with the proposed tool were well within 15% for the test farmer’s and controlled experimental fields in Gurgaon and Karnal districts of Haryana (India). In contrast to the actual relative wheat yield reductions of about 5.94, 19.93 and 22.07% on the controlled experimental fields in Sampla experimental station of Karnal district in Haryana, the proposed DSS predicted average wheat yield reductions of about 6.25, 18.03 and 38.61% under the irrigation treatments of (((6, 9 and 12 dS/m, respectively))) – this should possibly be moved up to paragraph above???.

Long term simulations with the proposed DSS showed that in contrast to the existing conjunctive water use practices, the long term Paddy season cyclic applications (Fig. 2) of CW: TW (associated with final root zone EC: 3.96 dS/m and ESP: 12.5%) and TW: CW (associated with final root zone EC: 3.8 dS/m and ESP: 13%) led to much lower soil root zone ECs and/ or ESPs than the other alternate Paddy season-conjunctive water use plans (associated with final root zone ECs ranging between 4.6 to 5.5 dS/m and ESPs ranging between 12.5 to 15%). It could be further observed that the existing Paddy and Wheat season-conjunctive water use practices led to about 24.5% and 13.6% relative Paddy and Wheat crop yield reductions respectively, at the end of the tenth year. In contrast to this, the Paddy season cyclic conjunctive water use option of CW: TW was associated with much lower paddy and wheat crop yield reductions (Paddy: 20% and Wheat: 12%) than those obtained with TW: CW (Paddy: 22% and Wheat: 13.3%). Blending of 50%CW with 50%TW was associated with final root zone EC of 3.7 dS/m; ESP of 15% and paddy and wheat crop yield reductions of 22% and 14% respectively. On comparing the above two cyclic modes of irrigation applications with the blending of 50%CW with 50%TW it was observed that cyclic application of CW: TW was better than the blending of 50% CW with 50% TW as it was not only associated with reduced root zone soil salinization but also reduced relative crop yield reductions. Other conjunctive water use practices were associated with much higher paddy (26 to 36%) and wheat (13 to 19 %) crop relative yield reductions.

In contrast to this the long-term impacts of alternate Wheat season conjunctive water use strategies (Fig. 3) showed that although alternate Wheat season-cyclic irrigation practice of 1CW, 1TW, 2CW was associated with slightly higher final root zone soil salinity/ sodicity values (EC: 4.8 dS/m; ESP: 13.1%) than the existing conjunctive water use strategy (EC: 4.1 dS/m; ESP: 13%) and much higher final root zone soil salinity/ sodicity values than 75%CW blended with 25%CW (EC: 2.84 dS/m; ESP: 11.4%) yet the final Paddy and Wheat crop relative yield reductions achieved with these three irrigation strategies (Paddy: 24-25%, Wheat: 12-13%) were not markedly different. Amongst the irrigation treatments with 2CW followed by 2TW (EC: 5.52 dS/m; ESP: 14.6%, Paddy: 35.3%, Wheat: 14.7%); 1CW, 1TW, 1CW, 1TW (EC: 3.8 dS/m; ESP: 13%, Paddy: 29%, Wheat: 13.3%) and 50% CW blended with 50% TW (EC: 3.7 dS/m; ESP: 14.8%, Paddy: 26.3%, Wheat: 14%), blending option appeared to be much better than irrigation treatment with 2CW followed by 2TW and only slightly better than cyclic application of 1CW followed by 1TW, both in terms of root zone salinization and paddy and wheat crop relative yield reductions. However even these irrigation treatments were not better than the existing cyclic conjunctive water use practice being followed on the test farm. Amongst the remaining irrigation treatments with 1CW followed by 3TW (EC: 5.5 dS/m; ESP: 12.5%, Paddy: 36.2%, Wheat: 19.3%) and 25% CW mixed with 75% TW (EC: 4.5 dS/m; ESP: 13%, Paddy: 28%, Wheat: 15%) also blending option appeared to be performing much better than the cyclic mode of irrigation. However even these irrigation strategies were not superior to the existing cyclic conjunctive water use practice. It was thus observed that the only way to substantially bring down the root zone soil salinization/ sodification and the paddy and wheat crop relative yield reductions (EC: 2 dS/m; ESP: 8.2%, Paddy: 21%, Wheat: 11%), through only Wheat season water management, was to apply all canal water irrigations.

Thus in contrast to the proposed alternate Paddy season conjunctive water use plan, which suggested application of two (i.e. 40 cm) additional canal water irrigations, the alternate Wheat season conjunctive water use plan suggested application of only one (i.e. 7.5 cm) additional canal water irrigation. Hence the proposed DSS suggested that for achieving (3-4%) higher long term returns and achieving reduced root zone salinizations/ sodifications, with the same crop rotation and minimum addition in the demand for the fresh water supplies, the farmer of the test farm may just alter his existing Wheat season conjunctive water use practice.

Conclusions

The present investigation led to the development of a well tested and validated decision support tool named IMPASSE which could give near-realistic estimates of soil root zone salinities/ sodicities, predict their future fate/ distribution, quantify impacts of varied levels of salt stresses under different combinations of land and water management strategies on relative crop yield reductions. It also allowed assessment of long term impacts of various conjunctive water use options on root zone soil salinization/ sodification and relative crop yield reductions thus enabling recommendations/ suggestions for best conjunctive water use/ irrigation practice for diverse areas.

References

Minhas PS and Gupta RK (1992). Quality of irrigation water assessment management. ICAR, Publication section, New Delhi.

Naresh RK, Minhas PS, Goyal AK, Chandan CPS and Gupta RK (1993). Conjunctive use of saline and non-saline waters. II. Field comparison of cyclic uses and mixing of wheat. Agricultural Water Management, 23,139–148.

Rhoades JD (1990). Strategies to facilitate the use of saline waters for irrigation. In: Water, Soil and Crop Management relating to use of saline water. FAO, Rome, pp.125-136.

Fig. 1 DSS-validation sites

Fig. 2 Long-term impacts of Paddy season conjunctive water use plans on Paddy & Wheat crop yield reductions

Fig. 3 Long-term impacts of Wheat season conjunctive water use plans on Paddy & Wheat crop yield reductions

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