L. D. van Rensburg

Reduction of evaporation from bare soil using plastic and gravel mulches and assessment of gravel mulch for partitioning evapotranspiration under irrigated canola

Partitioning of evapotranspiration (ET) into its components of evaporation (E) and transpiration (T) is difficult, yet important for managing unproductive and productive water losses under irrigated agriculture. A lysimeter experiment (Expt 1) was conducted on sandy Clovelly and sandy loam Bainsvlei soils in Bloemfontein, South Africa where plastic sheet and dolerite gravel mulches were applied to lysimeters to determine to what extent they restricted E from the soil surface compared with a bare soil control. No crops were grown in the lysimeters for Expt 1. Gravel mulch on Clovelly-filled lysimeters reduced E by 33% and by 41% for Bainsvlei-filled lysimeters compared with bare soil. Based on these results, lysimeter Expt 2 was undertaken on gravel mulched and unmulched bare soil lysimeters to assess the effectiveness of gravel mulch in partitioning ET into E and T using the Tanner and Sinclair (1983) method embedded in the soil water balance. In Expt 2, canola (Brassica napus L.) was grown in the lysimeters for 168 days. Gravel mulch had a significant effect on water use (WU) by suppressing the E component of ET, resulting in WU being on average 11% lower from gravel-mulched lysimeters than the unmulched lysimeters, and this translated to an improved average WU efficiency of 11.91kgha–1mm–1 for canola. Taken together, these results reinforce the potential for gravel mulch as a viable management option for soil water conservation, which is crucial for plant available water, a major limiting factor for plant growth in arid and semiarid lands.

Effect of Water Regime and Runoff Strip Width on Soil Water Balance for an Integrated In-Field Rainwater Harvesting

Integrating micro-flood irrigation with in-field rainwater harvesting (IRWH) was proposed and experiments were conducted in the 2007–2008 production season at Paradys Experimental Farm of the University of the Free State, South Africa. Three water regimes, dryland (DL), supplemental (SPI), and full irrigation (FI), were tested with 1, 2, and 3 m runoff strip width (RSW) to determine their effects on soil water balance components for the integrated IRWH. Four blocks with nine subplots were prepared for the 3 × 3 split plot factorial experimental design. Plots were 30 m long with a standard 1 m width. Site specific data were used to estimate rainfall-runoff and deep drainage functions. Soil water content (SWC) was measured with a Neutron water meter. Evapo-transpiration (ET) was partitioned into evaporation and transpiration using a β parameter. Deep drainage was the least significantly affected. Rainfalls not less than 24 mm had significantly higher gains on change in SWC for the 2 m and 3 m RSW. During dry spells these RSWs had significantly higher SWC deficit. For the vegetative and reproductive growth stages evaporation from the 3 m RSW constituted not less than 60% of ET compared to less than 40% from 1 m RSW. The 1 m RSW had significantly higher ET and T, irrespective of water regime with the highest values from irrigation treatments. The 1 m RSW can, therefore, be used with either FI or SPI to optimize soil water balance for the integrated IRWH water management.