Evaluating in-field rainwater harvesting with a sunflower - cowpea intercrop on a semi-arid ecotope in Limpopo province

Abstract

A field study was conducted during the 2007/2008 and 2008/2009 cropping seasons in order to evaluate the in-field rainwater harvesting (IR WH) production technique with sunflower (Helianthus annuus L.) X cowpea (Vigna unguiculata L.) intercrop. The IRWH is a special crop production technique that promotes runoff on 2 m wide no-till strip between crop rows and collects the runoff water in basins where it infiltrates into the soil profile. The IRWH was tested against the conventional tillage (CON). The study was carried out at the University of Venda (22°58' S, 30°26' E at 596 m above sea level) in Thohoyandou in the Limpopo Province of South Africa at the University of Venda- Shortlands ecotope. The potential for food production in the Limpopo Province is limited by low and erratic rain fall. The smallholder farmers in the province are the most vulnerable because they depend on dryland agriculture for livelihood. Crop yields in the province are typically low. It was therefore hypothesized that (i) IR WH will increase crop yields compared to the CON system, and (ii) cowpea intercropped as living mulches with sunflower will increase water use (WU), water use efficiency (WUE), PUE and grain yield of sunflower. The relationship between soil water content (8) and matric suction (h) or soil water release curve (SWRC) was obtained using the hanging water column (h :s 800 mm water). The drainage patterns as well the relationship between 8 and unsaturated hydraulic conductivity (K) for each diagnostic soil horizon was evaluated using the internal drainage method (IDM). Field saturated hydraulic conductivity (Ks) of each diagnostic soil horizon was determined using a double ring infiltrometer. Results from this study indicated that soil hydraulic properties were unique for each diagnostic horizon. The saturated hydraulic conductivity in the orthic A and structured Bhorizons was 30 mm h-' and 12 mm h-', respectively. The difference was largely attributed to the crumb microstructure observed in the orthic A-horizon. The results of the study also indicated that Shortlands the soil had good water retention properties as 19% (average for the profile) of the water was released between saturation and 8 kPa. It was further concluded that the plant available water (PA W) (267 mm) in the root zone was high and surpassed the soils tested for IRWH, making the University of Vend a-Short lands ecotope suitable for this production strategy. Rainfall on the ecotope was characterized using historical data (1983 - 2005) in Chapter 3. The statistical analysis of rainfall at the study site revealed that the annual rainfall was highly variable (CV of 315% for annual rainfall). Further analysis revealed that the probabil ity of receiving high rainfall amounts was low with small storms «20 mm) accounting for a large proportion of rainfall The field experiment to evaluate the IRWH with sunflower X cow pea intererop production is reported in Chapter 4. The experiment was laid out as a split plot design. Tillage systems formed main plots with cropping systems (CRS) as sub-plots. The treatments in the CRS consisted of a sole crop (sunflower or cowpea) and an intererop (sunflower x cowpea). The IRWH led to a significant (P < 0.05) increase in sunflower grain yield in the second season but cowpea grain yield was not influenced by tillage systems (TS). IRWH resulted in significantly higher water use (WU), water use efficiency (WUE) and precipitation use efficiency (PUE) of both crops compared to the CON system. The CRS had significant effects on sunflower grain yield in both seasons, but none on the cowpea grain yield. The effect of IRWH production on runoff was studied using a rainfall simulator in Chapter 5. Results of this study indicated that IRWH was superior in runoff generation compared to the CON system and it could supply I% of maize water requirements under the conditions of this ecotope. The Crop Yield Prediction for Semi-arid Areas (CYP-SA) model was applied to assess risk associated with IRWH on the ecotope. Using cumulated probability functions (CPFs), the results indicated that simulated sunflower yield was significantly influenced by initial profile water content. The IRWH was significantly better than CON at all levels of initial profile water content.