Permanent raised bed (PRB) farming systems combine several elements (e.g. reduced tillage, controlled traffic, stubble retention) of conservation agriculture. PRB systems have recently been introduced into irrigated areas but there is a lack of information on the agronomic and irrigation performance of these systems under these conditions. Three years of field research into the performance of irrigated PRBs was conducted in south east Queensland, Australia and north west Pakistan. These studies benchmarked the irrigation performance of variously sized PRB systems and explored the impact of bed renovation method (no tillage (NT), shallow cultivation (SC) and blade ploughing (BP)) on soil hydro-physical properties (bulk density, soil moisture storage, infiltration), irrigation performance, crop yield and input water productivity (WP). Irrigation management strategies to improve lateral infiltration and irrigation performance were also investigated.
The initial benchmarking study was conducted on two farms with clayey Vertisol (Australia) and three farms with sandy clay loam, Alfisol, (Pakistan) soils. The results showed low irrigation performance with excessive deep drainage potential. The irrigation application efficiency (Ea) on the clayey Vertisol and sandy clay loam was as low as 68% and 50%, requirement efficiency (Er) 96% and 77% and distribution uniformity (DU) 86% and 66%, respectively. However, the majority of the narrow beds (66 cm furrow spacing) were over-irrigated on the sandy clay loam while the wide beds (132 cm furrow spacing) were under-irrigated. Inappropriate renovation and bed furrow dimensions, sub-optimal irrigation management and poor lateral infiltration were the main factors likely to affect WP of irrigated PRBs.
The evaluation of bed renovation methods found that BP on the clayey Vertisol reduced (~6%) the average seasonal bulk density of the surface 0-30 cm compared to the NT treatment. BP was found to produce higher lateral infiltration into the beds than either NT or SC. Freshly applied BP and SC produced a higher (~23%) cumulative infiltration than NT but the effect was transient and was not significant in the following season when the PRB renovation treatments were not freshly applied. The SC beds slump more than the BP and NT beds. The soil water content in the beds indicated that there was increased water storage associated with BP. PRB renovation (i.e. SC or BP) was found to reduce irrigation performance on the clayey Vertisol when the irrigation was farmer managed. The volume of irrigation water applied to the fresh SC and BP treatments increased by up 13% and 55%, and Ea was reduced by up to 9% and 29% respectively, compared with NT during the 2010 wheat and 2011 corn seasons. NT produced a higher wheat yield and WP, but the lower wheat yield in the cultivated treatments was associated with poor crop establishment from a rough soil surface and inadequate seeder performance in this season. In the subsequent seasons, there was no significant difference in either the crop yields or WP between the bed renovation treatments.
Field trials investigating the effect of bed renovation on infiltration were conducted on both soils. Significant differences in the amount of water infiltrated and stored in the bed shoulder and bed middle were found for periods of wetting consistent with normal irrigation practices. This suggests that the general assumption of uniform soil moisture distribution across the beds is not valid for wide PRB systems. The three different renovation methods also significantly affected lateral infiltration, suggesting tillage may be used as a management tool to improve bed wetting in low infiltration soils and subsided beds.
Infiltration and soil-water movement were simulated using Hydrus 2D. The simulations were found to be well correlated with the measured field data and an evaluation for the three PRB renovation methods confirmed that NT had the slowest, and BP the fastest, potential to wet the bed middle of the wide PRBs. Increasing the furrow water head from 4 cm to furrow full of water was also shown to reduce wetting time by more than 30%. Lateral infiltration was poor in the sandy clay loam. The shortest wetting time (~15 hrs) required to wet the bed middle of a 132 cm wide bed to field capacity occurred when furrow full water head was applied to the BP treated bed. However, even this period of wetting may be difficult to achieve using current irrigation practices in Pakistan. The graphical model outputs developed can be used in developing guidelines to optimise bed width and irrigation management to ensure adequate lateral infiltration with different PRB renovation methods.
Strategies to optimise irrigation management and field design while ensuring adequate lateral infiltration into the bed middle were evaluated using the surface irrigation model SIRMOD. Optimising the inflow rate (Q) and the time to cut-off (Tco) for the particular field length was found to produce up to a 38% irrigation water saving compared with existing practices. However, the majority of the wide beds on the sandy clay loam required higher inflow volumes than currently applied, which increased Er but reduced Ea. Optimising furrow length indicated a further improvement in irrigation performance. Similarly, Q and Tco optimisation together showed up to 35% irrigation water saving and up to 33% improved Ea for the most sub-optimally managed BP treatment. Decreased furrow length improved irrigation performance for freshly renovated PRB, while irrigation performance of settled bed furrows tended to increase with increased furrow length. Relating optimum Tco to water advance to furrow tail end (Ta) was sensitive to furrow length, Q and soil infiltration functions. The decision support guidelines developed were helpful in improving existing irrigation performance under both soil conditions.
This research has shown that the existing irrigation performance of PRBs is often low and highly variable. A key constraint is the potential for poor lateral infiltration into the beds and inadequate wetting to the centre of wide beds. However, this work has also highlighted that the adoption of appropriate bed renovation methods (particularly BP) and irrigation management practices (e.g. Q and Tco) can substantially improve irrigation performance and input WP. Although the applicability of the specific decision support tools developed is restricted to the soil types and field conditions encountered, the general understanding and insight into the basic principles of performance optimisation and for agronomic and irrigation interactions is expected to be beneficial in refining understanding and the promotion of sustainable crop production under a wider range of environmental conditions.