PERIIMPLANTITIS Y MUCOSITIS PERIIMPLANTARIA

The regolith salt storage, measured at each borehole is presented in Table 4-9. The regolith is defined as the layer of loose, heterogeneous material which overlies hard rock (soil zone and vadose zone). The salt storage ranged between 15 t ha-1 and 922 t ha-1. The salt storage is dominantly a function of the depth of the profile as well as elevation, i.e. the regolith salt storage generally increased with decreasing ground elevation. Profiles located at Uitvlug (downstream in the catchment) exhibited elevated salt concentrations indicating that a salinity ‘hot spot’ may be located there. It should however be noted that this quantification technique incorporates a degree of uncertainty, which is mainly associated with the bulk density values. The estimates used in this quantification are from samples collected at a different site and at different depths.

4.3.4 Spatial Variability in Salt Storage

The point data of regolith salt storage was correlated with ground elevation (mamsl), the catchment TWI, and groundwater EC to identify relationships with which to interpolate the data. The coefficients of determination of the correlation analysis are presented in Table 4-10. The correlations were also evaluated using the Spearman’s rank correlation coefficient (Rs; Sorensen et al., 2005). Rs is a measure of the statistical dependance between two variables. If there are no repeated data values, a perfect Rs of +1 or −1 occurs when each of the variables is a perfect monotone function of the other. The sign of Rs indicates the direction of association between X (the independent variable) and Y (the dependent variable). If Y tends to increase when X increases, the Rs is positive. If Y tends to decrease when X increases, the Rs is negative. Salt storage did not correlate well with elevation. This is interpreted to be due to the fact that the borehole locations did not sufficiently account for elevation variations in the catchment, i.e. the borehole transects (Figure 4.3) were essentially drilled across three elevation zones/bands. Also, as the TWI incorporates elevation, the inadequate representation of elevation variation affected this correlation. Regolith salt storage correlated well with groundwater EC (mS m-1), exhibiting a coefficent of determination of 0.75.

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Table 4-9 Salt Storage (t ha -1) in the Sandspruit Catchment

Site Borehole

No

Soil Zone Salt Storage (t ha -1)

Soil Zone Profile Depth (m) Regolith Salt Storage (t ha -1) Regolith Profile Depth (m) Zwavelberg ZB001 8 3 28 11 ZB002 14 4 23 9 ZB003 8 2 52 11 ZB004 4 4 15 19 ZB005 4 4 16 13 ZB006 38 12 40 16 ZB006A 5 3 21 11 ZB007 4 2 75 15 ZB007A 8 2 97 12 Oranjeskraal OK001 4 2 18 8 OK002 4 3 79 29 OK003 10 4 39 29 Uitvlug UV001 8 3 349 69 UV002 168 3 922 15 UV003 55 2 519 22 UV004 211 16 341 30 UV005 45 18 97 53

Table 4-10 Correlation of Regolith Salt Storage (t ha-1) With Catchment Variables

Groundwater EC (mS m-1) * Ground Elevation (mamsl) TWI r2 0.75 0.41 0.07 Rs 0.72 -0.76 -0.21

* Groundwater samples collected in September 2010

The correlation between groundwater EC (mS m-1) and regolith salt storage (t ha-1) may be described by the equation:

salt storage (t ha-1) = 0.3269 (groundwater EC mS m-1) + 1.4292 (4.7) This correlation was used to interpolate salt storage across the catchment (Figure 4.5), by making use of additional historic groundwater EC data that were gathered from the National Groundwater Database (NGDB) of the DWA. According to Bennetts et al. (2006)this strong correlation is expected as saline groundwater may be a result of dryland salinity/high soil salinity. Interpolated regolith salt storage ranged between 3 t ha-1 and 674 t ha-1. The data also indicate that storage increases with decreasing ground elevation, i.e. in a north-easterly direction. Salt storage is expected to be lower in the hilltops, due to salt leaching and higher in the valleys due to salt accumulation. The spatially-averaged regolith salt storage in the Sandspruit catchment is 110 t ha-1. Due to the interpolation procedure and the historic nature of the groundwater EC data, these results incorporate uncertainty. They should thus not be considered as absolute values, but rather as estimates of salt storage.

Even though these results may not be taken as absolute values, the identification of areas of high salt storage also holds important implications for water resources management and land use planning. It enables resource managers to make informed decisions in terms of vegetation

110 distribution/planting strategies and the locations for the implementation of salinity management measures

Figure 4.5. Interpolated regolith salt storage (t ha-1) in the Sandspruit catchment.

4.3.5 Salt Output

The total annual export of salt from the catchment was calculated using the catchment discharge and streamflow salinity datasets (Equation 4.5). The results are presented in Table 4-11. During the period of observation the salt output from the Sandspruit catchment ranged between 12 671 t a-1 and 21 409 t a-1. The salt output is expected to be dominantly a function of discharge, i.e. rainfall amounts.

Table 4-11 Total Salt Output (t a-1) from the Sandspruit Catchment

Year Rainfall (mm) Discharge (mm) Salt Output (t a-

1₎

2007 655 32 16 890

2008 519 27 12 671

2009 444 37 21 409

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4.3.6 Catchment Salt Balance

The catchment salt balance, i.e. salt output/input (O/I) ratio is presented in Table 4-12. The rate of change in mass of the system (dM/dt) and the SBI/SER indicate that the Sandspruit

catchment is in a state of salt depletion, i.e. salt output exceeds salt input.

Table 4-12 The Salt Balance of the Sandspruit catchment Year dM/dt (t a-1) SBI/SER 2007 - 13 206 4.6 2008 -9 752 4.3 2009 -18 912 8.6 2010 -12 872 8.5

4.4 Conclusions

The TSI to the catchment from rainfall ranged between 1727 and 3684 t CA-1. The salt input was calculated as a function of rainfall amount as variable rainfall chemistry data were not available. Topsoil samples gathered during borehole drilling may generally be regarded as non-saline, exhibiting an ECe < 200 mS m-1. However, two samples gathered at Uitvlug,

which is located in the downstream parts of the catchment (Figure 4.1) exhibited elevated ECe (800 – 1600 mS m-1) and may thus be classified as very saline. This provides evidence

for the influence of topographic controls on salinisation processes. The salt storage in the regolith ranged between 15 t ha-1 and 922 t ha-1 and a salinity “hot spot” was identified in the lower reaches of the catchment. The point data of regolith salt storage were correlated with ground elevation (mamsl), the catchment TWI, and groundwater EC to identify relationships with which to interpolate these data. Regolith salt storage correlated well with groundwater EC (mS m-1), exhibiting a coefficent of determination of 0.75. Interpolated regolith salt storage ranged between 3 t ha-1 and 674 t ha-1, exhibiting generally increasing storage with decreasing ground elevation. The average regolith salt storage in the Sandspruit catchment is 110 t ha-1. During the period of observation the salt output from the Sandspruit catchment ranged between 12 671 t a-1 and 21 409 t a-1. The rate of change in salt mass of the system and the SBI/SER indicated that the Sandspruit catchment is in a state of salt depletion, i.e. salt output exceeds salt input. The input data and parameters required for calibration and validation of a semi-distributed hydrosalinity model have been calculated.

4.5 References

ACWORTH, R.I. and JANKOWSKI, J., 2001. Salt source for dryland salinity - evidence from an upland catchment on the Southern Tablelands of New South Wales. Australian Journal of Soil Research, 39, pp. 12-25.

ALLISON, L.E., BERNSTEIN, L., BOWER, C.A., BROWN, J.W., FIREMAN, M., HATCHER, J.T., HAYWARD, H.E., PEARSON, G.A., REEVE, R.C., RICHARDS, L.A. and WILCOX, L.V., 1954. Diagnosis and improvement of saline and alkali soils. Agricultural Handbook No. 60 edn. United States Department of Agriculture.

BENNETTS, D.A., WEBB, J.A., STONE, D.J.M. and HILL, D.M., 2006. Understanding the salnisation processes for groundwater in an area of south-eastern Australia, using hydrochemical and isotopic evidence. Journal of Hydrology, 323, pp. 178-192.

112 BUGAN, R.D.H., 2008. Hydrosalinity fluxes in a small scale catchment of the Berg River (Western Cape), University of the Western Cape.

BUGAN, R.D.H., JOVANOVIC, N.Z. and DE CLERCQ, W.P., 2012. The water balance of a seasonal stream in the semi-arid Western Cape (South Africa). Water SA, 38(2), pp. 201-212. CLARKE, C.J., GEORGE, R.J., BELL, R.W. and HATTON, T.J., 2002. Dryland salinity in south-western Australia: its origins, remedies and future research directions. Australian Journal of Soil Research, 40, pp. 93-113.

COX, J., DAVIES, P. and SPOUNCER, L., 1999. Water and soil degradation in the Keynes Catchment, South Australia. Technical Report 40/99. Australia: CSIRO Land and Water. CROSBIE, R.S., HUGHES, J.D., FRIEND, J. and BALDWIN, B.J., 2007. Monitoring the hydrological impact of land use change in a small agricultural catchment affected by dryland salinity in central NSW, Australia. Agricultural Water Management, 88, pp. 43-53.

DE CLERCQ, W.P., JOVANOVIC, N.Z. and FEY, M.V., 2010. Land use impacts on salinity in Berg River water. Report No K5/1503. Pretoria: Water Research Commission.

DWAF, 1986. Management of the water resources of the Republic of South Africa. Pretoria: Department of Water Affairs and Forestry.

DWAF, 1996. South African water quality guidelines (second edition). Volume 4 agricultural use: irrigation. Pretoria: Department of Water Affairs and Forrestry.

FEY, M.V. and DE CLERCQ, W.P., 2004. Dryland salinity impacts on Western Cape rivers. Report No 1342/1/04. Pretoria: Water Research Commission.

FLÜGEL, W., 1995. River salinity due to dryland agriculture in the Western Cape Province, Republic of South Africa. Environmental International, 21, pp. 679-686.

GILFEDDER, M., WALKER, G. and WILLIAMS, J., 1999. Effectiveness of current farming systems in the control of dryland salinity. Australia: CSIRO Publishing.

JOLLY, I.D., DOWLING, T.I., ZHANG, L., WILLIAMSON, D.R. and WALKER, G.R., 1997. Water and salt balances of the catchments of the Murray-Darling Basin. Technical Report 37/97. Australia: CSIRO Land and Water.

JOLLY, I.D., WILLIAMSON, D.R., GILFEDDER, M., WALKER, G.R., MORTON, R., ROBINSON, G., JONES, H., ZHANG, L., DOWLING, T.I., DYCE, P., NATHAN, R.J., NANDAKUMAR, N., CLARKE, R. and MCNEIL, V., 2001. Historical stream salinity trends and catchment salt balances in the Murray_darling basin, Australia. Marine and Freshwater Research, 52, pp. 53-63.

LEROTHOLI, S. 2005. The role of salinity as an abiotic driver of ecological condition in a rural agricultural catchment. MSc edn. Grahamstown, South Africa: Rhodes University. PECK, A.J. and HURLE, D.H., 1973. Chloride balance of some farmed and forested catchments in southwestern Australia. Water Resources Research, 9, pp. 648-657.

113 PECK, A.J. and HATTON, T., 2003. Salinity and the discharge of salts from catchments in Australia. Journal of Hydrology, 272, pp. 191-202.

PEGRAM, G.C. and GORGENS, A.H.M., 2001. A guide to non-point source assessment. Report No TT 142/01. Pretoria: Water Research Commission.

RICHARDS, L.A., 1954. Diagnosis and Improvement of Saline and Alkali soils. Agricultural Handbook No. 60. USA: USDA.

SAMUELS, D. 2007. Hydraulic properties of the vadose zone at two typical sites in the Western Cape for the assessment of groundwater vulnerability to pollution. M.Sc edn. Bellville: University of the Western Cape.

SLAVICH, P.G. and PETTERSON, G.H., 1993. Estimating the electrical conductivity of saturated paste extracts from 1:5 soil, water suspensions and texture. Soil Research, 31(1), pp. 73-81.

SORENSEN, R., ZINKO, U. and SEIBERT, J., 2005. On the calculation of the topographic wetness index: evaluation of different methods based on field observations. Hydrology and Earth System Science, 2, pp. 1807-1834.

THAYALAKUMARAN, T., BETHUNE, M.G. and MCMAHON, T.A., 2007. Achieving a salt balance—Should it be a management objective? Agricultural Water Management, 92(1- 2), pp. 1-12.

VAN HOORN, J.W. and VAN ALPHEN, H.A., 1994. Salinity Control. In: H.P. RITZEMA, ed, Drainage Principles and Application. 2nd, Pubilcation 16 edn. Wageningen, The Netherlands: ILRI, .

VERHOFF, F.H., YAKSICH, S.M. and MELFI, D.A., 1982. An analysis of total phosphorous transport in river systems. Hydrobiologia, 91, pp. 241-252.

WHITE, R.E., 2006. Principles and practice of soil science: the soil as a natural resource. fourth edn. Victoria, Australia: Blackwell Publishing.

WILCOX, L.V., 1963. Salt balance and leaching requirement in irrigated lands. 1290. WILLIAMSON, D.R., 1998. Land degradation processes and water quality effects, water logging and salinity. In: J. WILLIAMS, R.A. HOOK and H.L. GASCOIGNE, eds, Farming Action-Catchment Reaction, the Effect of Dry-land Farming on the Natural Environment. Collingwood, Victoria: CSIRO Publishing, pp. 162-190.

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