Handbook for Irrigation of Wine Grapes in South Africa

FIGURE 3.14. Measuring infiltration rate in a vineyard near Vredendal by means of the constant head, double ring method.

Water infiltration rates can be near zero for compacted soils, or well in excess of 100 mm/h for well aggregated soils. Soil conditions, particularly sodicity, may reduce water infiltration (Myburgh & Howell, 2012b). Low infiltration rates lead to ponding on nearly level ground. In vineyards, infiltration problems are aggravated by compaction on the wheel track (Fig. 3.15). Low infiltration rates also result in water runoff on sloping ground, and may cause soil erosion. Therefore, poor infiltration generally causes inefficient use of irrigation water. In this regard, it is important that the application rate of the irrigation system does not exceed the water infiltration rate. Organic matter, particularly crop residues and decaying roots, promotes aggregation so that larger soil pores develop, allowing water to infiltrate more readily (Fig. 3.16). PERMEABILITY The downward movement of water within the soil is called percolation or permeability. Similar to infiltration, this flow rate depends on the pore space in the soil. Therefore, permeability also varies with soil texture and structure (http://passel.unl.edu/ pages/informationmodule.php?idinformationmodule=1130447039&topicorder=10). Permeability is generally rated from very rapid to very slow (Table 3.2). This is the mechanism by which water reaches the subsoil and rooting depth of plants. It also refers to the movement of water below the root zone. Water that percolates deep into the soil may reach a perched water table or groundwater aquifer. If the percolating water carries chemicals such as nitrates or pesticides, these water reservoirs may become contaminated. 3.9

66 CHAPTER 3 – WATER RELATED SOIL PROPERTIES