Metelerkamp (1974) argues that aggregate stability should be assessed at the scale of a meter square surface in order to be more representative of the surface, and for kinetic energy to ponding to be the measure of preference rather than observation of aggregate breakdown. Whilst such an approach is clearly preferable, there is still an interest in this present study in determining whether and to what degree rainfall simulation results can be correlated to quicker and cheaper qualitative and semi-quantitative tests.
In this study simple tests of kinetic-energy-free wetting were undertaken (Plates 4 and 5), for comparison with rainfall simulation results. As the latter combines wetting and energy effects, whilst the former the effect of wetting alone, a comparison between the two is one way of addressing the research question: ‘What is the relative importance of the wetting effects and energy effects of rainfall on runoff?’, which will also be addressed by comparing low and high energy (drop height) rainfall simulations at the same rainfall intensity. This, in turn, may assist in answering one o f the research hypotheses originating from the review of the cmsting literature; is kinetic energy the key control on cmsting and, in particular, seal formation in the study area? This question is also addressed in the present study by assessing the results, in terms of effects on
c) Hand sprayer and windshield and (d) in use on a crusted surface; note spray pattern, with increasing ponding at centre
fc vv' Ï
L
%
(clockwise from upper left):
a) Crust sampler designed. Standard depth marks etched on side b) Crust samples dry sieved into aggregate classes; here variations in colour and reflectance are assessed between class sizes and sites c) Finest portion after sieving are m ixed with rain and well (source o f rainfall sim ulator water) w ater as two treatments; clay expansion compared between sites and as a function o f water type
d) Emerson test; sudden immersion o f aggregates for aggregate stability; both immediate and longer term effects can be recorded
o
infiltration and runoff, of various degrees of crust degradation (‘trampling’ and puncturing; see Plate 6).
The particularity of crust formation, where rainfall is assumed to be the principal agent responsible, is the delivery of energy in a co-quanta together with a given volume of water - a raindrop - ideally, therefore, both wetting and energy effects should be examined in unison. As these two are linked for a given drop height, comparison must be made at different drop heights for a given wetting rate if their respective effects are to be isolated, effectively altering the wetting unit : energy unit {instantaneous kinetic energy) ratio, and comparing the resultant seal formation by way of infiltration and runoff, measured on a common scale, in this case cumulative energy. Crust formation can be assessed, amongst other possibilities, by the change in surface porosity, as has been done here by the use o f hand spraying (Plate 4), before and 24 hours after rainfall simulation. Monitoring of changes in surface characteristics (in terms o f rate of water acceptance) under natural rainfall over the rainy season, after drying, again by means of a handsprayer (Plate 7) is another approach to assessing the effects of rainfall on the surface, albeit in this case without being able to separate the wetting and energy effects. The results of these various approaches will be reported in chapters 7 and 8 .
(4.4)
Crust stability upon wetting: details of tests employed in this study
(4.4.1)
Stability upon sudden wetting (immersion) and issues of soil and water chemistry
In terms o f simple tests of crust response to sudden wetting, the most appropriate model was considered to be a version of the classic Emerson Dispersion Index (Emerson, 1967), which classifies a soil aggregate response to sudden immersion into eight classes depending on the degree of cloudiness of the water. This cloudiness is taken to indicate the degree of dispersion caused by the clay in the aggregate. The advantage of this test is that it measures a property which has been shown to be an important mechanism in seal formation (pore clogging with fine material dispersed under the impact of rainfall) and which is quick, cheap and easy to carry out. The index used in this present study is a modification of the Emerson Index, which been simplified to only
Plate 6
Surface treatm ents used in rainfall simulations
a) (top) ‘Nail board’ used to puncture and to roughen crusted surface
b) (middle) rem oving crust (M arigat) c) (bottom) crust removed and control (M arigat)
, j Jit
(clockwise from top);
a) Preparing reserve plots in case o f rains (Lameluk)
b) Plots o f soil removed from Lameluk to residence; Plots under natural rainfall, crust and crust removed, all replicated, and one set o f plots under a rainfall sim ulator screen at 3.5m, the same height as the
simulator, to assess the effect o f reduced kinetic energy under natural rainfall.
c) Hand sprayer being used to m onitor crust development on the plots
four classes and has been used specifically with respect to crusting rangeland soils (in Australia), but not specifically for indicating runoff potential (Tongway 1994).
With all these tests, the high clay content at many of the sites implies that the chemistry, as indicated fi*om the review in Chapter 3, o f the water used could have a significant effect on apparent dispersibility and hence sealing / relative runoff ranking. Ideally, therefore, rainfall simulations would be carried out either with water of a standardised chemistry (distilled) or with natural rainwater. The latter was attempted, having set up a rooftop collection system, but the volumes collected were too small or infrequent, and thus this was reserved for the validation tests (see below) and for water efficient simple tests. In the case of simple tests, samples are immersed in both rainwater and the source water used for the rainfall simulation and the same repeated for the clay dispersion test and handspraying of crust aggregate classes. It is believed that this will give an indication of the practical consequences of any chemical differences between natural rainfall and the water used in the simulations.
(4.4.2)