Likely Effects Of Climate Change on Runoff from Limpopo Basin - Botswana
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Transcript of Likely Effects Of Climate Change on Runoff from Limpopo Basin - Botswana
Likely Effects Of Climate Change on Runoff from Limpopo Basin -
Botswana
By
Prof. B. P. ParidaDept. of Environmental Science
University of Botswana
Background
Much of Southern Africa below Latitude 20o S is basically Semi-Arid
This includes BOTSWANA whereAnnual Rainfall ~ 250 – 500 mmEvaporation ~ 2000 mmTopography ~ Flat and Deep Sandy Soil
(These together limit the effectiveness of Rainfall)
Sources of Water
Rivers and Streams : Average runoff is very low~0.6 mm for the whole country> All endogenous rivers originating in Botswana are ephemeral with an average of 10–70 days of flow in a year.
Other Sources: Desalination, Water carriers to tap international rivers – Very Expensive
Groundwater :
Location of Boreholes in Botswana
Sources..(Groundwater..contd)
Wellfield Extraction Rate
in M m3
Recharge Rate
in M m3
Jwaneng N. 10 2
Orapa 5 0.2
Palla
(Source: BNWMP, 1991)
12 2.4
Water Supply..
Clearly Botswana is one amongst the water stressed countries of the world(where water use is more than 20% of their renewable water supply)
Botswana is highly dependent on its natural resource base : mining, livestock, wild-life based tourism, textiles, soda ash and agriculture
In Summary:
Water is the single most important constraint to the development of Botswana
The Limpopo Basin
The major source of water in the eastern Botswana and it is important because of its strategic value to Botswana, South Africa Zimbabwe and Mozambique.
Drains from a catchment area of about 80 000 km2 (~1/8 area of Botswana)
Approximately carry about 10 mm of surface water annually.
The Limpopo Basin
The catchment serves as the main source for Gaborone & rapid and ongoing expansion of southeastern Botswana.
In Botswana alone.. 4 dams viz: Gaborone, Bokaa, Letsibogo and Shashe have been built with a total capacity of 350 M m3 (against projected demand of about 325 M m3 by 2020)
Indicative Studies Reveal that:
A rise of 0.02 to 0.05oC per decade in Africa in the present century (IPCC)
(greatest over the interior of semi-arid margins of Sahara and Central Southern Africa.)
Limpopo basin is likely to experience between 5 to 15% decrease in rainfall, 5 to 20% increase in Evaporation, and 25 to 35 % decrease in streamflow. (Desanker and Magadza, 2001).
Main objective
To quantify the effect of climate change on the flow regime of Limpopo basin – hence develop flow scenarios.
to find by what percentage the runoff will either increase or decrease in the coming years?
what will be the trend in flow over the coming decades in this century?
Philosophy:
Rainfall ~ Runoff is a complex paradigm as it represents the catchment’s response which is a function of land use
So, any adaptation strategy that needs to be developed to overcome the effects of climate change, would primarily centre around a well structured catchment management strategy along with strategies for water use management.
PROPOSED METHODOLOGY
Use past climatic and flow information (monthly rainfall, PE, and flow data over the past 20-30 years including data on landuse) to carry out water balance, hence compute the annual yield.
Use the past data as above to develop an ANN model which can be used for flow predictions.
Use the generated climatic scenario to develop flow scenario for the Limpopo.
CONCEPT OF MULTI-CELL WATER BALANCE :P = AET + OF + ΔSM + ΔGWS +GWR + Interception
stream
Impervious
OF
P
I
AET
Infiltration
SM
GWS
Percolation to G/water
GWR
Data Collection:
Rainfall Data: Monthly data at 37 stations (1971 – 1998)
Evaporation Data: Monthly data at 4 stations (1971 – 1998)
Flow Data : Monthly data at 1 station (1978 – 1998)
Soil Data : Field Capacity of major soil types in the basin as per the soil atlas.
Procedure for Multi-cell Water Balance: Long term average rainfall values and
evaporation values are superimposed on the soil map and the catchment is then divided into 75 cells – each cell basically representing uniform rainfall, evaporation and soil type.
For each cell and for a given year runoff is computed. (Thornthwaite and Mather, 1957)
For each year total runoff from the catchment is computed at the outlet by adding up flows from the 75 cells.
Total runoff / Total rainfall = Runoff coeff.
A TYPICAL WATER BALANCE COMPUTATION TABLE ( Field Capacity = 125 mm )
JAN FEB MAR
APR MAY
JUN JUL AUG
SEP OCT NOV
DEC
TOTAL
P 103 33 21 12 32 0 0 12 12 68 122 69 484PE 139 80 86 92 68 52 62 79 118 141 120 135 1172
P-PE -36 -47 -65 -80 -36 -52 -62 -67 -106
-73 2 -66
APWL -102
-149
-214
-294
-330
-382
-444
-511
-617
-690
0 -66
SM 55 37 22 12 9 6 3 2 1 0 125 73
ΔSM -18 -18 -15 -10 -3 -3 -3 -1 -1 -1 125 -52
AET 121 51 36 22 35 3 3 13 13 69 120 121
DEFICIT 18 29 50 70 33 49 59 66 105 72 0 14 565
SURPLUS
0 0 0 0 0 0 0 0 0 0 123 62
TARO 31 16 8 4 2 1 0 0 0 0 123 62
RO 15 8 4 2 1 1 0 0 0 0 61 31 123DET 16 8 4 2 1 0 0 0 0 0 62 31
Average Rainfall in Limpopo Catchment system between 1971 and 1998
y = -3.108x + 6556.2
0.00
200.00
400.00
600.00
800.00
1970 1975 1980 1985 1990 1995 2000
Years
Ave
arg
e R
ain
fall
(m
m)
Runoff Coefficient for Limpopo Catchment system between 1971 and 1998
y = 0.0049x - 9.255
00.10.20.30.40.50.60.7
1970 1975 1980 1985 1990 1995 2000
Years
Ru
no
ff C
oef
fici
ents
CUSUM Analysis of Rainfall Values in the Limpopo Catchment
-400.00
-200.00
0.00
200.00
400.00
600.00
800.00
1970 1975 1980 1985 1990 1995 2000
Years
CU
SU
M R
ain
fall
Val
ues
Runoff Coefficient in % and Rainfall for the Limpopo Catchment between 1981 and 1998
0100200300
400500600700
1980 1985 1990 1995 2000
Years
Ru
no
ff C
oef
fici
ents
(%
)R
ain
fall
(m
m)
CONCEPTUALISATION OF ARTIFICIAL NEURAL NETWORK BASED ON A
BIOLOGICAL NEURON
Cell body
Nucleus Axons
Dendrites Sunapses
Nucleus
Axon
Dendrides
Components of a neuron
HOW DOES THE ANN WORK ??
Processing element
Weights Wn Inputs Xn
Output path
Transfer Sum
Xo
X1
X2
X3
Xn
Wo
W1
W2
W3
W4 . . .
The basic structural functioning of a neuron
A MULTI-LAYERED FEED FORWARD NEURAL NETWORK WITH ONE HIDDEN LAYER
input layer
Hidden layer
Output layer
Output array
A multi-layer feedfoward neural network with one hidden layer
Conclusion
Using scenario of monthly rainfall and evaporation values from the GCM / RCM models (AF 07), scenarios on likely runoff from the Limpopo basin by the above two techniques will be created.
Co-operation needed:
Monthly rainfall and evaporation data scenarios for the Limpopo basin in the next 30 – 50 years will be necessary (GCM /RCM / Empirical / Other climate Model).
Formulation of appropriate catchment management strategies as possible adaptation strategy.