Investigations of soil cracking and preferential flow in a weighing lysimeter filled with cracking clay soil
Introduction
Deep drainage (DD) of irrigation water can impose a variety of effects on irrigation management. Irrigation water that becomes DD is lost for the growing plants and therefore reduces the application efficiency (Smith et al., 2005). The occurrence of DD potentially increases leaching of agrochemicals, such as various forms of Nitrogen, which constitutes a loss of nutrients from the farming system and potentially degrades the quality of receiving aquifers and surface waters (Bronswijk et al., 1995, Silburn and Montgomery, 2001). DD reaching the saturated groundwater zone can lead to rising water tables, which, once the capillary fringe of the ground water gets close to the surface and starts to evaporate, can severely increase soil salinity (Jorenush and Sepaskhah, 2003). Worldwide ∼1.5 million hectares of irrigated agricultural land is lost to salinity each year (Foley et al., 2005). At the same time, some DD can be beneficial in salinity management as it drains accumulated salts out of the top soil (Vervoort et al., 2003).
Fine grained cracking clay soils are prevalent in irrigation agriculture in many parts of the world. Due to their high nutrient content and water holding capacity they are highly productive soils. Because of their high clay content and thus low permeability, DD of irrigation water in these soils was long assumed minimal (Hearn, 1998, Vervoort et al., 2003). However, in recent years evidence of DD in cracking clay soils became widely accepted, yet estimates of its magnitude still vary greatly (Silburn and Montgomery, 2001, Smith et al., 2005, Acworth and Timms, 2009). For instance in a field investigation detected almost instantaneous arrival of irrigation water to a depth of 1.2 m as a response to flood irrigation on cracked soil.
Irrigation literature provides different management recommendations for irrigation and DD management on cracking soils. Mitchell and van Genuchten (1993) highlighted the advantage of flood irrigation, to make use of the high infiltration rates before crack closure, while Chen et al. (2002) recommended the use of low intensity sprinkler irrigation to avoid DD and leaching through soil cracks. Smith et al. (2005) emphasized that DD could be controlled by ensuring that the irrigation applications do not exceed the soil moisture deficit and Bethune (2004) recommended drying out fields after the irrigation season to prevent that water added by winter rains exceeds the soil moisture deficit and causes deep drainage. However, shrinkage cracks that form during drying of a soil could also favour the occurrence of DD due to the high importance of preferential flow through cracks in cracking clay soils (Bronswijk, 1988). It is known that preferential flow through soil cracks can cause rapid transport of irrigation water, solutes and agrochemicals through the unsaturated zone (Kosmas et al., 1991, Harris et al., 1994, Bronswijk et al., 1995, Lin and McInnes, 1995, Weaver et al., 2005), hence lowering the soils capacity of storage, adsorption and transformation of potential pollutants and greatly reducing irrigation efficiency.
To optimize irrigation management an improved understanding of the hydrology of these cracking clay soils is needed. In this study the process of soil crack formation and preferential flow was investigated in detail in a cracking clay soil in a weighing lysimeter. This controlled environment enabled the study of preferential flow and drainage in a highly cracked soil column under simulated strong rainfall and flood irrigations to determine the relationship between crack dynamics, bulk soil moisture content and macropore flow.
Section snippets
Methods and materials
The presented experiment took place over a time period of ∼5 years and consisted of three major parts, which are described in detail below. The investigated soil column was set up between September 2002 and February 2003. After a drying period of 30 month, a first set of measurements was carried out in 2005, followed by a further 20 month of soil drying and a second set of measurements in 2007.
Results
In November 2005, all cracks that were large enough to fit the 6 mm fibreglass tube of the videoscope were found to penetrate the entire soil profile of 450 mm. At two out of 20 insertion locations the lower 50 mm of the crack were filled with loose soil aggregates, presumably fallen in from above. The angle in which the cracks extended downwards, altered with up to from the vertical. In addition to the vertical crack network, near horizontal shrinkage planes were observed that intersected
Discussion
Based on the results in this study there are two interesting observations to be noted in regards to flow dynamics in cracking soils: Firstly, that drainage out of the soil column was not observed during the first three water applications and secondly that the drainage water that was collected after application 4 did show signs of preferential flow. As the observed crack dynamics play an important part in explaining these processes, they will be discussed before the flow dynamics are examined.
Conclusions
Crack dynamics and preferential flow were investigated during six irrigation events on an initially very dry and cracked soil in a weighing lysimeter. No drainage occurred out of the soil column during the first 3 out of 6 irrigation events, even though substantial surface runoff into the cracks occurred and, at least initially, soil cracks provided an uninterrupted flow path through the entire soil profile. This shows that lateral infiltration of macropore flow into the soil matrix was
Acknowledgments
This study was funded by the Cotton Catchment Communities CRC and the Cotton Research and Development Cooperation. The lysimeter was set up by Dr. Justin Bell.
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