CHAPTER 1 INTRODUCTION
1.7 Outline of document
4.2.5 Bottom boundary condition
4.11
Whilst in the unsaturated zone, water flow occurs mainly in the vertical direction.
However, in the saturated zone, water moves in a three-dimensional pattern, according to the hydraulic head gradients. The bottom boundary of the one-dimensional SWAP model is either the bottom of the unsaturated zone or the upper part of the saturated zone, and can be described by the:
• groundwater level or soil water pressure head as a function of time,
• specific bottom flux as a function of time, or
• bottom flux as a function of groundwater level (Van Damet al., 1997).
4.3 Sensitivityofsoil water balances modelled with SWAP to different input parameters
Wesselinget al. (1998) quantified the sensitivity of the SWAP model outputs to the changes in the process parameters for different scenarios. The results of the sensitivity analysis showed that 95 % of the variance of all the outputs could be explained by variance of the bottom flux. In general the influence of the crop factors used in the simulations and the preferential flow on the outputs were surprisingly low, whereas the upper and lower boundary layers were very important. The leaf area indices strongly determined the soil evaporation and crop transpiration whereas the lateral drainage was sensitive to the surface water levels. The effect of the secondary channels in the drainage systems is negligible compared with the influence of the primary channels. In terms of the soil water, the CPU time requirement for SWAP execution is insufficient to
complete simulations with low saturated hydraulic conductivities. The maximum groundwater level is strongly related to the surface water level, the minimum
groundwater levels to the leaf area indices, soil physical properties and surface water levels, whereas the average groundwater level is strongly dependent on the primary drainage system.
Van Dam (2000) also performed a sensitivity analysis with SWAP but focussed on the relative transpiration4and relative salt storage changes5• Van Dam (2000) found that the relative transpiration and relative salt storage changes were less sensitive to a change in the rooting depth, than the crop factor. A 50%reduction in the rooting depth caused the relative transpiration to change from 0.93 to 0.89 and relative salt storage to change form 0.14 to 0.10. However, a change in the crop factor of only 25%increased the relative transpiration by 0.06, and changed the relative salt storage from 0.14 to - 0.48. The effect ofa 33 % and a 16 % decrease in the Boesten and Stroosnijder soil parameter and saturated soil water content respectively, lead to an increase of 0.02 and decrease of 0.02 in the relative transpiration respectively. Some of the conclusions by Van Dam (2000) were that accurate data on crop factors and soil hydraulic functions are needed for reliable water and salt balances, and that the stress due to water shortage is affecting plant growth more than stress due to high salinity. However, the results suggested that for the specific research area no accurate rooting depth data were required.
4.4 Advantages and disadvantagesofthe SWAP model
SWAP can be used for investigating a range of different conditions, from alternative flow and transport concepts, laboratory and field experimental analysis and evaluation of management options with respect to field scale water and solute movement (Van Dam, 2000). Applications of SWAP includes the fields of ecology, desalinisation, design of drainage systems, irrigation scheduling, hydrological base for nutrient and pesticide transport, estimation of crop yield, analysis of surface water management to
~The relative transpiration can be defined as the ratio of the cumulative actual crop transpiration to the cumulative potential crop transpiration.
SThe relative salt storage change can be defined as the ratio of the change in salt storage of the soil profile over a certain time span to the initial salt storage of the soil profile.
detennine soil water flow, evaporation, crop growth, drainage, heat transport and/or solute transport and more (Wesseling et aI., 1998; Van Dam, 2000).
Disadvantages of SWAP identified during this modelling exerCise included the inability to model overlapping growing seasons, and simulate understorey evaporation.
The model further requires the use of high speed computers and even then cannot perform simulations for soils with low saturated hydraulic conductivities.
4.5 Application of techniques
This chapter concludes the description of the theory on the techniques (Chapters 2 and 3) and model (Chapter 4) applied in the research to determine the impact of different vegetation types on the soil water balance. The following chapter describes the research sites and the application of the techniques and model discussed in Chapters 2 to 4, to the research sites. That is followed (Chapters 6 to8)by a presentation of the results on the application of these techniques and model to a grassland and anE. viminalis site.
CHAPTER 5
MATERIALS AND METHODS
5.1 Introduction
The soil water balance approach was used to test the hypothesis of the study: whether a change in vegetation, from grassland toE. viminalis, will increase the total evaporation and decreases the soil water storage. Itis hypothesised that a change in vegetation from grasshmd toE. viminalis trees will reduce the drainage of water beyond the root-zone and into the mine workings, over the short- and long-term.
5.2 Simplified soil water balance
The simplified soil water balance at a field scale is given by:
P=ET
±
t::.S+Q
+ DwhereP is the precipitation, ET the evaporation (the sum of soil evaporation,
transpiration and interception), L1S the change in the soil water storage, Qthe runoff and D6the drainage beyond the root-zone, all components having the unit mm.
The soil water balance equation (Eq. 5.1) can be re-arranged to solve for the drainage term:
D
=
P - ET±t::.S - QAssuming the precipitation7 at two sites with different vegetation types is the same, the difference in the soil water movement below the root-zone (D) at the two sites can be attributed to the differences in the total evaporation (or the components therefore), soil water storage and runoff. Therefore, on a suface with a gentle slope and negligible
6The drainage below the root-zone refers mainly to soil water movement below the root-zone due to
r
avity. .. . ' . ,,,The precipitatIOnISassumed to be equal to the ramfall for the grassland andE. viminalissites.
5.1
5.2
runoff, differences in the drainage beyond the root-zones of grassland andE. viminalis trees are assumed to be due to differences in the plant soil water relations (total evaporation, soil water storage).
5.3 Conditions of site