Optimising runoff to basin ratios for maize production with in-field rainwater harvesting

[摘要] Food production in semi-arid areas principally depends on the availability of water. Consequently, improving rainwater productivity and modifying the available energy for unproductive water losses is an important and necessary step towards promoting rainfed agriculture in dryland farming. It has been convincingly argued that water management strategies on rainfed semi-arid areas, including in-field rainwater harvesting (IRWH) deserve considerable attention. However, integrated studies of water and energy balance on the IRWH technique in particular in optimizing runoff to basin area ratio and mulching levels (ML) was not comprehensively appraised. Therefore, in this thesis, the two main research questions concern: (i) what is the optimal runoff to basin area ratio to sustain maize crop yield? and (ii) how do the microclimatic conditions change under wide and narrow runoff strip length (RSL)? Field experiments were conducted (2007/08 and 2008/9) on the Kenilworth Bainsvlei ecotope, associated with high evaporative demand of 2294 mm per annum and relatively low and erratic rainfall (528 155.6 mm). Topographically the area had a gentle slope (< 1%) with reddish brown in colour (Amalia family) a fine sandy loam texture soil, thus was classified as a Bainsvlei form. The soil is regarded as very suitable for dryland agriculture, because it is deep (2000 mm) and drains freely in the top and the upper sub-soil. So the study was performed by quantifying and evaluating the soil-crop-atmosphere parameters. In the first part of the thesis, the soil water balance components and different efficiency parameters were assessed. In the second part of the thesis, the micrometeorological variable profiles within and above the maize canopy for the heat and water vapour exchange processes were characterized. Furthermore, comparison of available energy for evapotranspiration (ET) was evaluated for both wide and narrow runoff strips through the quantification of energy balance components. A multiple regression model was developed to predict in-field runoff by combining the effects of rainfall event characteristics and surface treatments. From the results of runoff-rainfall (RR) ratio a lower efficiency was observed from full mulch covered wide runoff strip length (RSL-3) i.e. only about 4% of the rainfall, while the highest mean RR was about 27% from bare, narrow RSL- 1. From the estimation of rainfall canopy interception (RCI) it was revealed that the highest interception was in the range of 4.5% to 9.0% of the precipitation. The RCI capacity of a maize field under IRWH reached a plateau at about 0.5 �?0.6 mm for narrow RSL and 1.0 �?1.1 mm for wide that would be evaporated eventually from the canopy. Furthermore the cumulative Es (ΣEs) was evaluated as influenced by both mulch ('dry-mulch) and shading ('green-mulch) effects. Thus, the proportion of water loss by Es from seasonal rainfall is about 62%, 64% and 66% in the bare treatments and as low as 28%, 30% and 32% for full mulch cover treatments under full shade, (FC), partial canopy shaded (PC) and unshaded (UC) respectively. This implies that, reduction of runoff and evaporation losses through surface treatments can promote improved water use efficiency, of the stored available water in the root zone and thus, enhance yield. The final grain yield decreased slightly as an order of increasing the length of the runoff strip. The performance of the harvest index (HI) was slightly variable among the treatments due to more water for yield being collected from bare plots than mulch covered plots. The higher mulch conserves much water by suppressing the soil evaporation. In expressing grain yield per unit ET (WUEET) and transpiration, Ev (WPEv) the RSL-2 m and RSL-1.5 m at lower mulch cover showed significant higher values than RSL-1 and RSL-3 treatments. However, the transpiring water for yield and unproductive evaporation losses more under IRWH should be evaluated in terms of micrometeorological profile characterization and available energy. With regard to micrometeorological variables, the growth stage had a strong effect on the vertical profiles of climatic variables. In wide runoff strips lapse conditions extended from lowest measurement level (LP) to the upper middle section (MU) of the canopy and inversion was apparent at the top layer (UP) of the canopy. The reason for the extension of temperature inversion into the upper part of the wide RSL canopy was as a result of higher air movements compared to narrow strips. From this result it was confirmed that the effect of wind on water vapour removal decreased downward from wind flow within the canopy. This had an influence on the resistance of the boundary layer and canopy and soil surface resistance. This is a clear indication that wide strips supply more drying power to respond to evaporative demand of the atmosphere compared to narrow strips. From the measurement of profiles within and above the canopy, it was suggested that, the presence of local advection in the wide runoff strips of IRWH could be a common phenomenon causing variations in water vapour removal under the heterogeneous nature of IRWH tillage system. Thus, profile characteristics within and above a plant canopy are playing a great role in determining the vapour pressure deficit and consequently, can explain the ET rate. Therefore based on micrometeorological measurements, results indicated that the latent heat (LE) was dominant and higher in wide compared to narrow runoff strips (RSL) under both dry and wet conditions. However, sensible heat (Hs) showed lower values on wide runoff strips during wet conditions due to the advective effect of the runoff area. Thus, the wide runoff strip with a higher basin leaf area ratio (BLAR) of 2.43 had higher ET and used more energy in evaporating water than the narrow runoff with a lower BLAR of 1.42. Wide runoff strips converted the higher available energy more efficiently into a higher biomass production. During wet days, the wide RSL used more than 70% of the available energy for evapotranspiration, while the narrow RSL response to the available energy (63%) was stronger during dry compared to wet days. In general the wide and narrow RSL used the available water and energy differently during dry and wet conditions under IRWH system. From this experiment finding, important implications were described such as better yield obtained from narrow RSL-1, however RSL-1.5 and 2 m with minimum mulch cover gave higher water productivity compared to narrow RSL-1 and wide RSL-3. On the other hand when quantifying and evaluating the cause behind the effect of available energy, the wide RSL converted available energy more efficiently into higher biomass production than the narrow RSL. Therefore, this challenge should be addressed on the basis of an integrated approach to water and energy resources in order to develop comprehensive management strategies. Furthermore, for improved rainwater use management strategies, it is recommended to link an integrated approach of water and energy resources with crop growth simulation models. The application of the crop models could be important by incorporating a range of planting dates and densities along with the selection of surface treatment management strategies

[发布机构] University of the Free State