SA Grain October 2017

Oktober 2017

Salt-affected soils and waterlogging

on irrigation schemes

t the very first South African Irrigation Congress held in

1909, much concern was expressed at the extent of salt-

affected soils and the sediment content of water supplies

(Kanthach, 1909). At the National Irrigation Symposium

82 years later, Scotney and Van der Merwe (1991) had the same

concerns and said that the long-term viability of soil and water re-

sources is in jeopardy. Major threats to these resources result from,

among others, salinity, sodicity and waterlogging.

A review of about 3 000 soil irrigation reports at the ARC-

Soil, Climate and Water, revealed that soils free of limitations

for sustainable irrigation are limited in extent in South Africa. How-

ever, it appears that waterlogging, salinity and sodicity affects only

10% to 18% of the area under regular irrigation in South Africa.

This is much lower than experienced in many countries, because of

the strict emphasis placed on the potential for waterlogging, salinity

and sodicity and its prevention in the selection criteria for irrigated

soils in South Africa in the past. Another advantage is the generally

good water quality that has historically been available for irrigation

in South Africa.

Currently the salinity and sodicity of South African waters are on

the increase due to mining, urban, industrial and agricultural devel-

opments and the re-use of water resources. Irrigated agriculture is

not only at the receiving end of water quality deterioration, but also

a contributor to water quality deterioration experienced in many

rivers. The use of this water poses a future threat for soils on South

African irrigation schemes where leaching is limited.

It is evident from information available that the degree of degrada-

tion varies considerably between irrigation schemes and also over

time within the same irrigation scheme in South Africa. An increase

in salinity and sodicity normally coincide with hydrologically dry

years with below-average runoff and an increase in waterlogging

during hydrologically wet years.

Satellite images to quantify and identify

salt-affected soils and waterlogging

National monitoring of waterlogging and salt accumulation are a

high priority, but currently no verified methodology is available to

undertake this task. A recently completed Water Research Com-

mission project by researchers from the ARC-soil, Climate and

Water and Stellenbosch University sought to determine the po-

tential of various data sources and techniques for monitoring wa-

terlogging and salt accumulation and to quantify the current level

of waterlogging and salt accumulation and monitor changes over

time at the appropriate scale on irrigation schemes in South Africa

(Nell

et al

., 2015).

Remote sensing is the practice of deriving information about the

earth’s land and water surfaces using images acquired from an over-

head perspective, by employing electromagnetic radiation in one

or more regions of the electromagnetic spectrum, reflected or emit-

ted from the earth’s surface.

The challenge with using remote sensing for identifying and de-

lineating waterlogged and salt-affected areas is that they are local

manifestations and can only be differentiated from unaffected areas

by taking its context (surrounding area) into consideration. For in-

stance, an affected area within a wheat field will have very different

spectral properties to an affected area in a vineyard, while the latter

will have a very different spectral response compared to an affected

area within a bare/fallow field.

The occurrence of salt accumulation and waterlogging in generally

mall patches in South African irrigation schemes poses additional

challenges.

FOCUS

Irrigation

Special

DR PIET NELL,

ARC-Soil, Climate and Water

STUDY AREA

AFFECTED

ADJUSTED

NAME

HA*

Vaalharts

26 434

27 033

414,7

1,57

848,9

3,14

Loskop

38 831

40 867

887,1

2,28

2 344,7

5,74

Makhathini

4 312

4 624

138,5

3,21

361,1

7,81

Olifants River

11 284

11 911

224,6

1,99

664,9

5,58

Tugela River

27 384

28 244

1 477,3

5,39

2 102,8

7,44

Breede River

29 129

30 188

1 396,8

4,80

2 215,3

7,34

Sundays River

18 608

18 832

528,2

2,84

740,5

3,93

Limpopo River

8 681

8 805

468,1

5,39

564,0

6,40

Douglas

22 748

23 445

1 483,3

6,52

2 124,0

9,06

MEAN

20 823

21 550

779,8

3,78

1 329,6

6,27

* Area adjusted by adding abandoned fields

TABLE 1: SUMMARY OF THE AREAS AFFECTED BY SALT ACCUMULATION AND WATERLOGGING.