Consideraciones éticas y de rigor Consideraciones éticas

According to a review by Doerr et al. (2000), the main hydrological impacts of soil water repellency are reduced infiltration capacity, increased overland flow, spatially localised infiltration, a change in the three-dimensional distribution and dynamics of soil moisture, enhanced stream flow responses to rainstorms, and enhanced total stream flow.

A number of studies have confirmed that water repellency compromises the infiltration capacity of a soil (DeBano, 1971; Wallis et al., 1990b; Wallis et al., 1991) and that increased overland flow occurs (Crockford et al., 1991; Witter et al., 1991). There are a very limited number of papers which examine the consequences of soil water repellency in New Zealand hill country pastoral systems. Of note is a study connecting ‘dry patch syndrome’ and soil

water repellency in Hawke’s Bay hill country pastures (Deurer et al., 2007) which prompted an analysis by Müller et al. (2010) of the effects of water repellency on pasture growth at two permanent pasture sites at Whatawhata in the Waikato and six sites at Maraetotara in the Hawke’s Bay. The former location was used to test the effect of repellency on water infiltration and solute transport using soils with high and low organic carbon contents, while the latter location was used to test the effect of repellency on pasture production by installing pasture cages on 3 hydrophobic and 3 control sites.

Using disc infiltrometers with ethanol and water at Whatawhata, the authors found that repellency appeared to reduce the permeability of the soil by a factor of 6 and 20 in the low and high organic carbon content soils, respectively. These values were consistent with other values for New Zealand soils reported in the literature (Wallis et al., 1990b), and suggest that repellency is likely to enhance both overland flow and localised infiltration. Intact Whatawhata soil columns were used in leaching experiments to measure transport of the herbicide 2,4-D with results indicating reduced sorptivity of this compound in the high organic carbon (and highly repellent) soil. The low organic carbon soil displayed significantly higher overall filtering efficiency. These results suggest that the enhanced localised infiltration caused by repellency reduces the opportunity for solutes to enter soil macro- aggregates where degradation would normally take place and that the soil’s normal filtering efficiency is compromised by soil water repellency.

Experimental results from their Hawke’s Bay sites (Müller et al., 2010) showed a 50 % reduction in pasture production over 4 months in the ‘dry patches’ compared to the wetter areas surrounding them, with the ‘dry patches’ making up about 30 % of the pasture area. The authors suggested that ‘dry patch syndrome’ may lead to an estimated 30-40 % loss in pasture production. These figures are not definitive however, since only three replicates were used, which is insufficient for a statistical analysis and may be the reason why the authors seemed not to have attempted this. The Müller et al. (2010) paper also implies that the positioning of cages at the Maraetotara site was by selection rather than being randomised, indicating the potential for bias. Other factors which may also influence pasture production appear not to have been measured, such as pasture species, density, and soil fertility.

While not specifically examining water repellency in hill country, there are a number of papers which allude to the possibility of water repellency having an effect on their research outcome. In an attempt to develop a pasture production model based on a daily soil water balance and where pasture growth was proportional to actual evapotranspiration, Moir et al. (2000a) suggested that the over prediction of pasture growth at their Whareama sites on hill country in the eastern Wairarapa, particularly during summer and autumn, may have been due to the soil surface becoming water repellent. While this may have been a factor, the authors acknowledge that other reasons would also have accounted for the slow recovery of dead or desiccated pasture after rainfall, such as the reliance of pasture growth on germination or re-growth from buried stolons.

In their study on the development of a soil water balance model for sloping land, Bircham and Gillingham (1986) acknowledged the role of repellency during the rewetting phase of the soil where “ ... on a dry, steepland soil, often only the surface few millimetres of the profile will be re-wetted during a rainfall event, almost regardless of the intensity of rainfall.” The authors accommodated this observation into their 4-layer model by controlling the rate of water entry into the top layer so that “ ... duration rather than intensity of rainfall tends to control the rate of soil rewetting.” This was achieved by imposing a minimum time of 3.3 days to allow the top layer to reach field capacity if the moisture content of that layer was less than 0.68 of field capacity. While it is highly likely that water repellency does compromise the infiltration rate at the surface layer, the authors do not justify the threshold soil moisture value at which the throttling takes place nor the period of time over which it occurs. The result of the imposition of their throttling process was an underestimation of soil moisture values during September and October because of the inability of the sub surface layer to be rewetted until the surface layer had reached or exceeded field capacity. During these months, high evapotranspiration rates often kept moisture contents of the surface layer below that of field capacity and prevented the rewetting of the layer below it.

A number of papers have described the interaction of lime application and soil moisture responses in hill country. During et al. (1984) observed a rapid increase in soil moisture in the top 25 mm of soil after an application of 3 t ha-1 of lime on over-sown pasture at Whatawhata. This effect was less marked on steep than on easy slopes during autumn.

Work by Jackson and Gillingham (1984) showed a soil moisture advantage to liming except at very low soil moisture levels. This advantage was most pronounced during late summer and early autumn and the mechanism proposed by these authors was that the application of lime relaxed hydrophobic conditions promoted by organic matter formed by herbage senescence in summer. Morton et al. (2005) however, reported only small increases in soil moisture in response to the application of lime at a site near Waipawa, suggesting that the lime-soil moisture-repellency relationship is complex and that more parameters need to be monitored in order to better understand the soil surface chemistry response to lime applications.