A Del­i­cate Bal­ance

The Oka­vango Delta

Indwe - - Contents - Text: Michael Mur­ray-Hud­son: Se­nior Re­search Fel­low, Oka­vango Re­search In­sti­tute, Univer­sity of Botswana & Olivier Dau­teuil: Di­recteur de Recherche au CNRS, Univer­sité Rennes 1 / www.the­con­ver­sa­tion.com Im­ages © iS­tock­photo.com

The Oka­vango Delta in north­ern Botswana is a mo­saic of wa­ter paths, flood­plains and arid is­lands. The delta sits in the Oka­vango river basin, which spans three African coun­tries: An­gola, Namibia and Botswana. Be­cause it’s an oa­sis in a semi­arid area, it hosts a rich ar­ray of plants and at­tracts a huge va­ri­ety of wildlife.

As a unique ecosys­tem, in 2014 the Oka­vango Delta was placed on UNESCO’s World Her­itage list and it is an iconic tourist des­ti­na­tion, which gen­er­ates 13% of Botswana’s GDP. But it’s a frag­ile nat­u­ral area. It’s con­trolled by de­for­ma­tions of the Earth’s crust over a long time (thou­sands to mil­lions of years) and by an­nual wa­ter flows and evap­o­ra­tion. The size of the flooded delta from year to year varies be­tween 3,500 km² and 9,000 km² be­cause of weather fluc­tu­a­tions which con­trol its wa­ter sup­ply.

Any change to the pro­cesses that form the delta will have an im­pact on wildlife and lo­cal eco­nomic ac­tiv­i­ties. Its grassy flood­plains are food for graz­ing an­i­mals in the dry pe­riod. Losses of this habi­tat will cause de­clines in wildlife and live­stock. It’s there­fore im­per­a­tive to un­der­stand what cre­ates and sus­tains the delta for the fu­ture man­age­ment of the sys­tem.

We have con­ducted sev­eral stud­ies that cover how the Oka­vango basin was formed and the way dis­solved chem­i­cals are with­drawn from the delta’s sur­face.

The dy­namic his­tory of the Oka­vango Delta’s wa­ter­ways and flood­plains tells us that the in­ter­play be­tween ge­ol­ogy, wa­ter and plants makes the delta re­silient, but vul­ner­a­ble. Some im­mi­nent changes are ex­pected that are of con­cern. One is higher tem­per­a­tures, which will boost evap­o­ra­tion and tran­spi­ra­tion. An­other is the pump­ing of wa­ter for ir­ri­ga­tion in Namibia. Both of these changes will re­duce the wa­ter needed to sus­tain the delta’s flood­plains.

An Oa­sis

The Oka­vango Delta is a gen­er­ally flat area which is un­der con­stant change with phases of flood­ing and dry­ing. A va­ri­ety of

ge­o­graph­i­cal and nat­u­ral pro­cesses have formed it and sus­tain it.

It’s in a de­pres­sion which was cre­ated by fault lines cut­ting the Earth’s sur­face. This means wa­ter flows into it. The fault lines are cre­ated by the spread of the East African Rift – a ma­jor frac­ture, cre­ated over mil­lions of years, which crosses the eastern part of Africa.

The ori­gin of the is­lands in the delta is at­trib­uted to two mech­a­nisms: the con­struc­tion of ter­mite mound spires; and for­ma­tion of el­e­vated ridges where for­mer chan­nels de­posited sand. Both act as the start­ing point for veg­e­ta­tion to take root.

The wa­ter sup­ply comes from the Cubango and Cuito rivers in An­gola. This reaches the delta be­tween March and June and peaks in July. There’s also lo­cal rain­fall in the Oka­vango area from Novem­ber to Fe­bru­ary (about 450 mm a year) which adds to this.

About 98% of the wa­ter that goes into the delta is even­tu­ally lost through evap­o­ra­tion and plant tran­spi­ra­tion, when wa­ter moves through the plant and evap­o­rates from leaves, stems and flow­ers.

Even though the sub­trop­i­cal sun gen­er­ates in­tense evap­o­ra­tion, the delta’s wa­ter is fresh, (not salty). This is sur­pris­ing be­cause wa­ter sam­ples from the mid­dle parts of the is­lands have very high chem­i­cal and salt con­cen­tra­tions. This chem­i­cal con­cen­tra­tion oc­curs in thou­sands of is­lands.

The rea­son the wa­ter is fresh is that trees on the edges of the is­lands have cre­ated a bar­rier of nat­u­ral fil­ters be­tween the in­ner part of the is­lands and the floodplain.

Pos­si­ble Changes

The Oka­vango Delta is con­tin­u­ally be­ing shaped by com­plex in­ter­ac­tions of nat­u­ral pro­cesses. If some­thing hap­pens to change the bal­ance of these pro­cesses, it could desta­bilise the sys­tem.

The most im­por­tant dy­namic for the delta is in­flow­ing wa­ter. The two main rivers in An­gola, the Cubango and the Cuito, join to form the Oka­vango River, which feeds the delta. These two rivers are hy­dro­log­i­cally quite dif­fer­ent. The Cubango, to the west, first flows rapidly down steep, nar­row paths char­ac­terised by in­cised val­leys, rapids, wa­ter­falls and val­ley swamps. The Cuito, to the east, with shal­low val­leys and large flood­plains, gets its wa­ter from ground­wa­ter seep­age.

The hu­man ma­nip­u­la­tion of these rivers – in the form of dams and ir­ri­ga­tion – will af­fect the wa­ter flow and change its an­nual dis­tri­bu­tion. Both of these form part of cur­rent and fu­ture de­vel­op­ment plan­ning in An­gola and Namibia.

A de­crease in wa­ter sup­ply will af­fect the veg­e­ta­tion growth and wildlife. An in­crease in wa­ter would in­un­date the is­lands and could dis­solve the salts at the cen­tre of them, re­leas­ing chem­i­cal el­e­ments that would change the wa­ter qual­ity.

In ad­di­tion to de­clines in wa­ter flow in­duced by global warm­ing and hu­man ac­tiv­i­ties, ground de­for­ma­tion is also hap­pen­ing be­cause of shift­ing con­ti­nen­tal plates. This could change the paths of the wa­ter flow­ing by chang­ing the ground slopes. Mea­sure­ments of ground de­for­ma­tion with Global Po­si­tion­ing Sys­tems dis­plays re­veal very slight changes in lo­cal slopes that can mod­ify the paths of the wa­ter flow­ing to the delta.

To sus­tain the Oka­vango Delta, it’s im­per­a­tive that man­age­ment in­te­grate all the com­po­nents of the sys­tem. All gov­ern­ments are in­volved and must in­te­grate sci­en­tific ex­per­tise, from up­stream catch­ment to down­stream delta.

The Oka­vango Delta is con­tin­u­ally be­ing shaped by com­plex in­ter­ac­tions of nat­u­ral pro­cesses. If some­thing hap­pens to change the bal­ance of these pro­cesses, it could desta­bilise the sys­tem.

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