WATER CONSERVATION TECHNIQUES ON SMALL PLOTS IN SEMI-ARID AREAS TO ENHANCE RAINFALL USE EFFICIENCY, FOOD SECURITY, AND SUSTAINABLE CROP PRODUCTION
Dec 2003
1176/1/03
EXECUTIVE SUMMARY
The project had three main objectives: (a) to study quantitatively within the in-field rain water harvesting (IRWH) system, different combinations of mulching techniques aimed primarily at reducing evaporation from the soil surface, soil fertility aspects with the focus on nitrogen, and the sustainability of the system; (b) to develop the capacity of three previously disadvantaged technical assistants; and (c) to transfer the technology to the developing farmers and to the Department of Agriculture. The first and second objectives were achieved concurrently by means of field experiments at Glen and on farmers' fields near Thaba Nchu, the technical assistants gaining invaluable skills and experience by being intensively involved in all the relevant activities. The crops grown were maize, sunflower and beans. The third objective was achieved by means of the demonstration plots on farmers' fields, and by numerous information days, lectures and demonstrations in the villages, all held in close cooperation with officials of the Department of Agriculture.
In the executive summary of a preceding WRC report entitled "Optimizing precipitation use efficiency for developing farmers with limited access to irrigation water", further research on eight aspects was recommended. The current project has dealt with seven of these in an appropriate way. This demonstrates the foresight of the Water Research Commission in terms of efficient value adding, in this case to the benefit of developing farmers.
Each ecotope was characterized in detail to lay the foundation for efficient extrapolation of all experimental results to other ecotopes where field experiments have not been done.
Runoff measurements on the Glen/Bonheim ecotope, converted to long-term predictions by appropriate equations, indicated that where the 2 m runoff strip in the IRWH system was left bare, covered by stones (60 - 70 % cover), or organic mulch, the amount of water harvested annually in the basins was predicted to be 196, 130 and 62 mm, respectively, more than with the conventional (CON) treatment. It needs to be kept in mind that these results represent the respective systems without a crop. They represent conditions present during early spring rains and while the crop is small. With a mature crop much rain will be intercepted, tending to equalize runoff from the different treatments to some extent. This is only one of the factors which causes measured yield differences between treatments to be disproportional to the predicted amounts of water gained in the basins by harvesting from the runoff strip. The runoff/sedimentation studies revealed that basins could be expected to become silted up in approximately 12, 21 and 75 years with bare, stone covered and organic mulch covered runoff strips, respectively. This information is important in relation to the sustainability of the IRWH system. Of primary importance is the fact that with this system, since ex-field runoff is reduced to zero, ex-field soil loss is also zero. In contrast, topsoil losses of around 0.6 mm per year were measured on conventionally tilled plots on long-term runoff experiments at Glen and Pretoria. The advantage of IRWH with regard to sustainability is clear. Maintenance of the basins to ensure that their capacity does not fall below a critical value will be necessary, whatever treatment is implemented, to prevent them from overflowing during heavy storms
Four different IRWH systems and CON tillage were compared in field experiments at Glen and on farmers' fields near Thaba Nchu. The four IRWH treatments were: organic mulch in the basins with a bare runoff area (ObBr); organic mulch in the basins with organic mulch on the runoff area (ObOr); organic mulch in the basins with stones on the runoff area (ObSr); stones in the basins with organic mulch on the runoff area (SbOr). The crops grown were maize, sunflower and dry beans. An empirical stress model termed "Crop Yield Prediction for Semi-Arid Areas" (CYP-SA) was developed to enable long-term yield predictions to be made. The composition of the model for each crop is described in detail, together with validation results. Measured yields over three seasons in the field experiments, and predicted long-term yields using the model, showed the following: For all the crops on all the ecotopes the ObSr treatment was the best IRWH treatment, but generally only statistically significantly better than ObBr. The clearly defined superiority of IRWH over CON, already shown in the 1996 - 1999 WRC project, was confirmed. These results are strongly supported and eloquently described by the long-term, yield cumulative probability functions (CPFs) obtained with CYP-SA. The yield advantage with early January planting of the crops, compared to mid-December and mid-October planting dates, is also well described by the CPFs.
Measurements on the experimental plots showed that water losses by deep drainage were negligible during the three growing seasons. Since runoff losses were also zero on all the IRWH plots, the only cause of water loss was evaporation from the soil surface (Es). This was shown on the maize plots to be highest on the ObBr treatment, and similar on the other three treatments, amounting to 79 % and 74 % of the annual rainfall, respectively. The equivalent value for the sunflower plots was approximately 70 % for all the treatments. It is clear that in order to further improve precipitation use efficiency (PUE) above the current best values of 7.4 and 4.8 kg ha-1 mm-1 for maize and sunflower respectively, it will be necessary to find ways of suppressing Es still further. An initial study in this connection was made by calculating infiltration ratios for the basins versus the runoff area. Data for all IRWH treatments on the maize and sunflower plots was pooled and the ratios plotted against relative yield. A parabolic curve fitted the data well (r2 = 0.98) and the optimum infiltration ratio, associated with stones on the runoff area, was found to be between 1.6 and 1.9. This is a valuable result and more research with different kinds of mulch (e.g. gravels) could lead to further improvements in PUE.
The soil fertility study focused on nitrogen. Two levels were applied in the IRWH experiments: a recommended level (15 kg N ha-1), and a "high" level (70 kg N ha-1 for maize and 90 kg N ha-1 for sunflower). Where the available water (rainfall plus profile available water) was > 470 mm, sunflower responded positively to the high N level (significant at P = 0.05). Maize did not respond to the high N level, probably due to: (i) the lower available water levels (376 - 404 mm), and (ii) the relatively high nitrogen supply capacity for the crop-soil system of 58 kg N ha-1. Both crops responded to 15 kg N ha-1 compared to zero N. The study also revealed that the recommended N application of 15 kg N ha-1 needs to be slightly increased, depending on the mulch combinations and crop rotation systems, if agronomic and conservation sustainability is to be maintained in the long run.
Specific "indicators" used to monitor socio-acceptability of the IRWH technique included the following: community, mobilization, capacity building, empowerment, human well-being, self-reliance and community participation. These indicators showed that there is a strong movement towards building an active learning process in farmer groups and individual farmers lower down the hierarchy. When taking account of the large study area and huge number of end-users, much time and effort was invested in empowering the end-users, which is essential for long-term sustainability. The large number of farmer-managed trials (108 homesteads in 6 villages), and the 'training of trainers' programme, formed a solid basis from which further training and learning could be launched. The residents of villages who applied the IRWH technique at their homesteads during the 2002/03 season, varied between 1 and 46 families per community. From the first season (2001/2002) the homesteads in which IRWH were applied expanded from 6 in four villages to 108 homesteads in six villages. Four new communities implemented the IRWH technique at their own request. The residents of the different villages planted a variety of crops, some of which were never demonstrated to them, as part of their IRWH strategy to combat food insecurity. They planted maize, dry beans, watermelons, pumpkins, peas, carrots, beetroot, spinach, tomatoes and potatoes.
Short-term data indicate that the IRWH technique is far more sustainable than CON for this specific study area. Of all the IRWH techniques tested, ObSr has been shown to be the best, followed by SbSr, ObOr, SbOr and ObBr. In general, evaluation of the project in terms of the different sustainability criteria (agronomic productivity, risk - security, conservation techniques, economic viability and social acceptability) really needs to be done in the long-term. Only then could it be properly judged whether or not the project has succeeded in implementing sustainable land management practices (or best practice components thereof) among an acceptable percentage of the farmers. However, there are sufficient indications that they are moving towards the goals, objectives and outcomes of sustainable agriculture. If the ARC-ISCW perseveres with the approach of 'true participation' for a critical period of at least 5 to 10 years, there is no doubt that sustainability can be achieved.
Taking all the above mentioned results into consideration it can be concluded that IRWH will enhance PUE, promote food security and ensure sustainability. The ObSr treatment was overall the best treatment, followed by SbSr, ObOr, SbOr, ObBr and a bare basin with a bare runoff area (BbBr).
RECOMMENDATIONS, INCLUDING ADDITIONAL RESEARCH NEEDS