Water footprint of growing vegetables in selected smallholder irrigation schemes in South Africa

Crop water footprint (WF) is the volume of fresh water used to produce a certain crop in all the steps in the production line. The CROPWAT model was used to calculate crop evapotranspiration, differentiating green and blue water in Zanyokwe (ZIS), Thabina (TIS) and Tugela Ferry (TFIS) Irrigation Schemes. Green beans had the highest water footprint in all three irrigation schemes with 3 535.7 m3/t in TIS, 2 753 m3/t in TFIS and 2 407.6 m3/t in ZIS. Cabbage had the lowest water footprint. The highest water footprint for growing cabbage was 254.5 m3/t in TFIS, followed by 223.1 m3/t in TIS, and the lowest was 217.8 m3/t in ZIS. Green WF represented the highest percentage of water use at ZIS (50.5%), followed by blue water at 26.5% while grey water constituted 22.9%. At TFIS blue, green and grey water use was 23.1%, 56.7% and 20.2%, respectively. The differences observed in the WF of different crops and different schemes were attributed to the differences in weather and environmental characteristics. Green beans had the highest grey water footprint, i.e., 373 m3/t and the lowest was cabbage with 37 m3/t. Potato, spinach and tomatoes had footprints of 156 m3/t, 214 m3/t and 132 m3/t, respectively. For future research it is necessary to consider the possibility and trade-offs of shifting production of each crop to the places where it is most efficient, and to focus on blue water scarcity in each of the case study locationsKeywords: smallholder irrigation schemes, water footprint, evapotranspiration, water us

Rainfall intensity effects on crusting and mode of seedling emergence in some quartz-dominated South African soils

Predicted changes in rainfall intensity due to climate change are likely to influence key soil health parameters, especially structural attributes and crop growth. Variations in rainfall intensity will impact crop production negatively. It is therefore imperative to investigate the interaction between predicted increases in rainfall intensity and key soil health parameters, particularly in relation to soil structural attributes and plant growth. The objectives of this study were to determine the effects of rainfall intensity on soil crust formation and mode of seedling emergence in soils dominated by primary minerals. Soil samples were collected from the top 200 mm, air dried and then packed uniformly into plastic pots, which were perforated at the bottom. Three maize seeds of equal size were planted in a triangular pattern in each pot at a depth of 30 mm, after which the pots were pre-wetted by capillary. The samples were then subjected to simulated rainfall at 3 intensities, i.e., 30, 45 and 60 mm/h, for 5 min. Rainfall intensity significantly (P < 0.05) affected crust strength and mean emergence day (MED), but not emergence percentage (EMP) and shoot length (P > 0.05). The 60 mm/h rainfall intensity resulted in the highest crust strength and MED. The strength of crust for all three rainfall intensities was influenced by quartz content, soil organic matter, clay and hematite. Most seedlings emerged through cracks, which resulted in rainfall intensity having no significant effects on seedling EMP and shoot length. We concluded that any increase in rainfall intensity is likely to increase the severity of crusting in these soils. However, soils with extensive cracking are likely to have higher EMP and lower MED and more vigorous seedlings despite the strength of the crust. As a result, post-planting tillage methods that enhance crust cracking may be employed to enhance seedling emergence and growth in these soils.Keywords: climate change, crusting, mineralogy, penetration resistance, soil organic matte

Rainfall pattern effects on crusting, infiltration and erodibility in some South African soils with various texture and mineralogy

Rainfall characteristics affect crust formation, infiltration rate and erosion depending on intrinsic soil properties such as texture and mineralogy. The current study investigated the effects of rainfall pattern on crust strength, steady state infiltration rate (SSIR) and erosion in soils with various  texture and minerals. Soil samples from the top 0.2 m layer were exposed to 60 mm·h-1 simulated rainfall. The rainfall was applied either as an 8-min single rainstorm (SR) or 4 x 2-min intermittent rainstorms (IR)  separated by a 48 h drying period. Rainfall pattern significantly (p < 0.05) affected crust strength, SSIR and erosion. The IR resulted in higher crust strength and SSIR than SR. The effect of rainfall pattern on SSIR was mostly influenced by the primary  minerals, namely, quartz. Therefore, the predicted shift from long duration to short duration rainstorms due to climate change is likely to enhance crust  formation and soil loss in semi-arid areas such as the Eastern Cape Province of South Africa.Keywords: hydrology, penetration resistance, quartz, soil organic matte