Heemst, H.D.J., van

Penning de Vries, F.W.T.

English

van Heemst, H.D.J.

NARCIS 

Potential primary production of unirrigated land in the Sahel

Wageningen Yield 

POTENTIAL PRIMARY PRODUCTION 
OF UNIRRIGATED LAND IN THE SAHEL 
F.W.T. PENNING DE VRIES and H.D.J. VAN HEEMST * 
SUMMARY 
Primary production in arid and semi-arid regions· is often limited by available 
soil nutrients. The removal of this limitation will increase yields of grasslands 
and agricultural fields up to a level imposed by amount and distribution of 
precipitation and the water storage capacity of the soil. This study shov,•s that 
it is possible to compute this potential plant production to be between 0 and 
4,000 kg above-ground dry matter per hectare per year with 270 mm of rain, 
and 5,000 to 9,000 kg per hectare per year with 540 mm of rain, depending upon 
soil type, soil depth and rainfall distribution. 
INTRODUCTION 
To grow, plants need nutrients, water and light. 
In contrast to a frequently expressed opinon, it is 
often not water shortage that keeps the productivity 
of the vegetation low in semi-arid regions, but a poor 
availability of nutrients, particularly of nitrogen. 
Rainfall only triggers the growing season. In huge 
areas of Australia legumes have been introduced 
successfully to improve the supply of nitrogen in 
grasslands. This has stimulated the growth of 
grasses .and increased the productivity of cattle very 
substantially in the last decade, according to, among 
others the interesting book 11 Australian Grasslands", 
edited by R.M. Moore (1970). Based on such results, 
and on a preliminary inspection of soil chemical 
data (in the 11 Notice explicative", numbers 13, 14, 
16 and 24 to the ORSTOM "Carte Pedagogique" of 
Chad and Senegal, published in 1964 and 1965; G.J. 
Staring, pers. comm.) it is expected that appropriate 
legumes and Rhizobium strains can be found to 
increase the soil nitrogen supply in Sahelian natural 
grasslands and in agricultural fields, and thus to 
raise their primary productivity. A higher nitrogen 
availability increases both quantity and nutritional 
values of the vegetation. 
Yields may be increased by such measures up to 
a level imposed by precipitation. Plant transpiration 
and plant production are then closely related (de 
Wit, 1958). Since it is impossible to provide large 
(*) F.W.T. PENNING de VRIES : Department for Theo· 
:etical Production Ecology, Agricultural University, Wagen-
mgcn, NETHERLANDS; and H.D.J. VAN HEEMST : 
Institute for Biological and Chemical Research on Field 
Crops and Herbage, P.O. Box 14, Wageningen, NETHER· 
LANDS. 
areas with abundant water, the natural precipitation , 
determines the maximum productivity of grasslands 
and rainfed agriculture. The relatively small areas 
in warm and dry climates that can be irrigated may 
yield 5 to 10 times as much dry matter as unirrigated 
areas (de Wit, this volume). 
All animals require food and water. Water is 
taken from open sources if the food is too dry. 
Depending on the geographical location, current 
weather and animal species, both water and food can 
become scarce thus limiting subsistence and growth 
of animals. The water requirement per kg of dry 
food is from ten to a hundred times smaller than 
that of plants to produce the piomass. In principle, 
therefore, it seems easier to supply animals with 
sufficient water than plants, and that primary pro-
duction in semi-arid regions sets a maximum to 
animal productivity. In the long run, an important 
question will thus be: what is the maximum produc-
tivity of vegetation on unirrigatcd land in semi-arid 
regions? 
This question is attacked by Buringh, Van Heemst 
and Staring (1975) on a regional scale, and by Van 
Keulen (1975a) on a small field scale. The first 
study calculates the potential primary production of 
unirrigated land by multiplying the potential rate of 
growth of the vegetation by the smallest of two 
reduction factors, the first of which reflects topo· 
graphy and disturbing soil physical and soil chemical 
properties, and the second of which represents effects 
of water shortage. The potential growth rate of 
closed canopies on good soil with plenty of water 
and nutrients depends essentially on light intensity 
and temperature, and is remarkably similar for a 
wide range of agricultural and non-agricultural crops. 
Applying their method to Sahel conditions predicts 
maximum yields of 3,400 kg plant dry matter ha-l 
-323-
a·t the 250 mm isohyct and 7,100 to 8,400 kg dry matter 
ha-l with twice as much rain. 
Van Keulcn's approach combines knowledge of soil 
physics, agrometeorology, and plant physiology to 
keep track of the soil water balance and of plant 
arowth during the growing season. This model is 
described below, and was used to compute vegetation 
growth at sites in the Sahel with weather conditions 
typical at the 250 mm and 550 mm isohyets, and 
with 7 soil types, including a shallow sandy soil, a 
deep loamy soil, and a heavy clay soil. Results of 
these computations are presented. 
A simulation model of potential production on un-
irdgated fields · 
Van Keulen (1975a) divides the soil into 10 layers 
with identical properties, but with water contents 
and amounts of roots that may be different. The 
thickness of the layers increases from 2 em at the 
top, to 6 to 30 em at the bottom, such that the 
total depth of the profile is 50 to 180 em. Water is 
added to the first layer by infiltration. The amount 
infiltrated equals the amount of rain minus run-off, 
or plus run-on. If more water is supplied to the 
upper layer than can be held in it, as determined 
by the field capacity of the soil, the excess drains 
into the second layer. When this layer still contains 
som·e water, or if the excess of the upper layer is 
large, it also becomes saturated, and the excess drains 
into the third layer. Persisting rains or large run-on 
:may saturate the whole profile and eventually lead 
to drainage out of reach of roots. 
Evaporation from the soil surface draws largely 
on \Vater of the upper layers, and little on those 
kg dry matter. ha-1 
9000 
8000 . 
5000 -500 
4000 -400 
3000 300 
below 25 em. Potential soil evaporation is calculated 
according to the Penman formula, corrected for 
the radiation reflected from the , soil surface and 
that absorbed and reflected by the canopy. The 
actual rate of soil evaporation equals the potential 
rate, reduced by the outcome of an empirical for~ 
mula. Redistribution of water in the profile driven 
by water potential gradients is slow compared to 
water extraction by roots and evaporation, and is 
therefore tre~tcd in a very simple manner. 
Calculation of the vcget<.\tion transpiration is also 
based on the Penman formula, which is adjusted to 
the current leaf surface per hectare of soil surface, 
and in which the leaf resistance against transpira-
tion is included. Water lost by the canopy is 
extracted evenly over the rooted zone from all layers 
in which the water content still exceeds the wilting 
point. The rooting depth increases by about 1 em 
per day as long as the plants grow. 
There is a ratio of 100 to 300 between the rate of 
transpiration (kgH20/ha-ljday-l) of a canopy and its 
gross (kgC02/ha-1/day-1) assimilation. The actual 
value of this ratio depends largely on the current 
weather and is computed continuously in the model. 
Canopy transpiration divided by this transpiration 
ratio yields the canopy assimilation. Some of the 
assimilates are utilized in biochemical maintenance 
processes, and the remainder is multiplied in the 
model by a conversion factor of 0.75 to reflect the 
growth processes and to give the vegetation dry 
weight increase. The distribution of this dry matter 
over roots and shoots is related to the developmental 
stage of the plants by an empirical formula. It is 
assumed that 0.5 percent of the living vegetation dies ' 
II 
I 
\ ! 
\ i 
'I 
- ~ 
\ 
\ 
\ 
\I 0/0 ;fL 
o/ 1 1 0--- v 
2000 -200 
-9_-o- /r 1000 10U o-,o--o 
0 
April May 
50 
1 June 
/ 
100 
July 
/ 
r ----WP 
/ 
150 · 200 day number 
l August I September! October 
-324-
per day throughout the growing season. This rate 
increases to 10 percent per day once soil water is 
exhausted. Plants die also when their physiological 
development is completed and seeds arc ripe. The 
rate at which this stage is approached is a function 
of temperature. Seed germination starts when the 
upper soil layers are moist for about 5 consecutive 
days. The parameters and the relations to tempera-
ture, soil moisture and dc,:dopmcntal stage, \vhich 
in the model characterize the type of vegetation, 
apply to annuals, and more specifically to wheat and 
natural grasses. In terms of potential productivity 
and water use efficiency, there is remarkably little 
di(ference among such species. 
The simulation model is based on a fair knowledge 
of the soil and plant processes, and is quite reliable. 
Meteorological inputs for the simulation program 
are daily readings of maximum and minimum tempe-
rature, radiation, relative or absolute humidity, wind 
speed and precipitation. The simulation model 
repeats the computation of the rates of aU pro-
cesses over one day from the current status of the 
system and the actual weather, and integrates these 
until the end of the growing season has been reached. 
The model has been used to simulate vegetation 
growth in the Negev desert on deep, loamy soils. 
Precipitation . in this area near Beersheva on the 
250 mm isdhyet falls between October and April. 
In 3 consecutive years well-fertilized fields were 
harvested at regular time intervals and<'their soil 
moisture content was recorded. Results of. simula-
tions of these experiments agree with the observa-
tions within the experimental error of 15 percent in 
almost all cases. Such results were obtained without 
entering the never-ending procedure of adapting 
model constants of parameters other than weather 
data, to improve model behaviour (Van Keulen, 
1975b). 
Sahelian soils and weather 
Van Keulen's model applies to all warm arid and 
that plants in the Yegctation mature ncar the end of 
the period in which a moist soil allows growth. 
Sahelian soils are generally very sandy, although 
some contain much clay. In the model, soils arc 
characterized by their water content at field capa-
city (which corresponds to a water potential of 
-0,3 bar) at the permanent wilting point (-15 bar} 
and when air dry (about -1,000 bar). Numerical 
values for these water contents in different soils, and 
for frequently occurring soil depths were collected 
from various reports and are shown in table l ; the 
soil types 1, 2, 3 and 4 are estimated to cover about 
15 percent, 25 percent, 20 percent and 15 percent of the 
Sahel, respectively (G . .T. Staring, pers. comm.). 
Long-term monthly averages of weather data were 
taken from the World Survey of Climatology (Grif-
fits, 1972) for Gao at the 250 mm isohyet and for 
Mopti at the 550 mm isohyet, both in Mali. All 
factors but precipitation change slowly throughout 
the year, so that their daily values can be derived 
by interpolation. The summer months in Gao are 
somewhat warmer and drier than in Mopti. Inspec-
tion of the total precipitation in May, June, July, 
August, September and October in Gao (8, 23, 71, 
127, 38 and 0 mm respectively) and in Mopti (23, 
56, 147, 198, 94 and 18 respectively) and the· number 
of days with more than 1 mm of. precipitation (1, 
4, 8, 8, 4, and 0 in Gao; 2, 7, 11, 12, 8 and 2 in Mopti) 
indicates an average precipitation per rainy day of 
10 to 15 mm in the wettest months and 6 to 8 rnm 
.in the preceding and following months. This preli-
minary study assumes a regular distribution of ave-
rage showers within each month. It is realized, 
however, that an irregular rainfall distribution can 
be of critical importance for plant survival and plant 
growth, for run-off and deep drainage. The results 
presented demonstrate, therefore, the capabilities of 
calculation of potential production when the course 
of the weather is known or predicted, rather than 
giving exact predictions for production. 
semi-arid regions. This section specifies the data Potential primary production of unirrigated land 
needed to simulate conditions prevailing in the in the Sahel 
Sahel. . 
Of the plant properties, only the response of the Figures 1 and 2 present typical results ?f s:m':Ia-
rate of development to temperature is lowered, such tions of \Vater usc and dry matter production durmg 
kg dry matter ha -1 
6000 600 mm 
0 
April May 
Fig. 2 
FC 
0 -o--o 
.,....,..- 0--0 ,.,,.,. 0-0 0---- ,... .... :,._ ____ w p 
-.....r7r::_= o.P-o-o"' 
--
50 100 150 200 day number 
I June July I August !september! October Month 
-325-
I 
the growing season, using soil properties and weather 
data as inputs. For a luvisol of 180 em depth with 
540 mm rain, a top yield of 8,900 kg of above-ground 
dry matter ha-1 is computed (fig. 1) ; on cambisols · 
near Gao, highests yields arc calculated to be 3,300 
dry matter ha-1• · Figure 1 shows that, with the rain· 
fall distribution assumed for Mopti, only after some 
100 mm of rain is the soil moist enough to permit 
germination. The initial phase of exponential growth 
lasts about 40 days and ends when the canopy inter-· 
cepts almost all direct radiation. It is followed by 
p. phase of a slowly decreasing growth rate of 120 kg 
ha-ljday-1 in the beginning to 85 kg ha-1/day-1 at 
the end of the season. By the first weeks of October 
the vegetation is ripe and dies rapidly. 
Table 2 compares potential yields on the main 
Sahel soil types, and shows that all soils deeper than 
50 em can yield with 540 mm of rain 7,000 to 9,000 kg 
dry matter ha-ljyear- 1, except the heavy clay soil 
(type 4). This soil needs most water and time to 
become sufficiently moist for germination. Conse-
quently, it loses so much water by surface evapora-
tion that little remains for the plants, which is 
reflected by the low potential yield. On the sandy 
soils, roughly as much water is lost by deep drainage 
as is transpired by the vegetation; but in all cases 
considered, still more water evaporates from the soil 
surface. 
In Gao, the most productive soils are those with the 
lowest storage capacity (table 2), because the least 
water gets lost in the drying process of their upper 
layers between showers. On the other hand, a 
depth of at least 100 em is required to prevent 
considerable drainage. Some 150 mm of rain is 
Table 1 
The water content of different soils at field capacity, 
required before germination starts (fig.· 2). Once 
established, the crop grows at a rate of about 90 
ha-ljday-1 until the soil is dry or the plants have 
completed their life cycle. In the examples presented,_ 
this occurs after 50 to 65 days. The longer the vege-
tation grows, the higher the total biomass produced 
(table 2). 
One of the conclusions of these simulations is that 
at the 250 mm isohyet, even on the most productive 
soils, 70 or more percent of the precipitation :is eva-
porated from the soil (table 2). The soils with a high 
field capacity lose essentially aU water by evapora-
tion, and no growth occurs. Drainage is seldom im-
portant. When the vegetation grows, its transpiration 
rate is as high or exceeds the soil evaporation rate, 
but due· to the short growing season plants utilize 
only about one sixth of the total rainfall. Again it 
must be recalled that run-off and an irregular .rain-
fall distribution will modify these proportions by 
adding more water to deeper layers, from which 
evaporation is very slow. It will depend on the soil 
storage capacity and its depth how much water will 
drain out of the root zone and how much is available 
to the vegetation. It is obvious, however, that on 
many soils a considerable loss of water by either 
surface evaporation or drainage is unavoidable. If 
plants have completed their life cycle before 
aU available water is used, as in the example of 
Figure 1, the water is well protected against evapora-
tion in deep layers during the dry months, and 
becomes again available to plants in the next season. 
In deep. soils, carryover of water to the next growing 
season can be important to reduce the effect of 
water shortage in a following dry year. 
at the permanent wilting point and when air dry 
Soil type Field capacity Wilting points Air dry Depth -
(g.cm-3) (g.cm-3) (g.cm-3) - (em) 
Cambisol (type 1) 0.08 0.025 0.015 100, 180 
Luvisol (type 2) 0.14 0.06 0.04 50, 100, 180 
Luvic-arenosol (type 3) 0.20 0.10 0.04 150 
Fluvisol, gleysol, planosol, ver-
tisol (type 4) 0.36 0.24 0.15 100 
Table 2 
Primary productivity and water use on 7 soils and in two weather regimes* 
Soil Total production Maximum live Growing season. Transpiration, soil 
Precipitation biomass duration evaporation, •stora-
(kg/ha-l) ge and deep drai-
Type Depth (kg/ha-l) (days) nage as a percen-
(em) I'J tage of precipitation 
1 180 8,600 6,600 112 31, 41, 4, 24 
1 100 7,700 6,000 108 28, 41, 0, 31 
2 180 8,900 6,800 112 33, 48, 10, 9 
536 mm 2 100 8,700 6,700 112 32, 48, o. 20 
(Mopti) 2 50 7,100 5,300 96 25, 48, 0, 27 
----
3 150 8,500 6,600 112 32, 65, 3, 0 
4 100 5,200 3,800 88 ]9, 83,- 2, 0 
1 180 3,300 2,700 64 18, 79, 3, 0 
1 100 3,300 2,700 64 18, 74, 3, 5 
2 180 2,800 2,200 56 16, 92,- 8, 0 
267 mm 2 100 2,800 2,100 56 16, 88,- 4, 0 
(Gao) 2 50 2,300 1,800 52 B, 84,·- 1, 4 
3 150 100 100 1 0, 112, -12, 0 
4 100 0 0 0 0, 115. -15, 0 
Presented arc the total amount of biomass produced above ground, the maximum amount of live biomass. du;ing th: 
growing season, the duration of the growing season and the distribution of prccipitati.on o.ver c~nopy tr~nsptratwn, sotl 
surface evaporation, storage in the soil (the difference between the water contents m mtd-Apnl and mtd-Octobcr), and 
drainage out of reach or the root system. 
* Fn·qucntly occ-•1ring soil ckpths arc given in the last column. 
-326-
BIBLIOGRAPHY 
1. BURINGH, P., VAN HEEMST H.D.J. and STARING 
G.J. - Computation of the absolute maximum food 
production of the world. A report by the Department 
of Tropical Soil Science, Agricultural University, 
Wagcningen, 1975. 
2. Griffits, J.F. - World Survey of Climatology : Vol. 10, 
Climates of Africa. Elsevier, London, 1972. 
3. KEULEN, H. Van - Simulation of w~ter use and her-
bage growth in arid regions. Simulation Monographs, 
Pudoc, Wageningen (published : spring 1975), 1975. 
4. KEULEN, H. Van. - Evaluation of models. In Ecolo-
gical models, with emphasis on grassland models. G. 
Arnold and C.T. De Wit, eds., Simulation Monographs, 
Pudoc, Wageningen (published : summer 1975), 1975. 
5. MOORE, R.M. - Ed. Australian Grasslands, A.N.U. 
Press, Canberra, 1970. 
6. WIT, C.T. de. - Transpiration and crop yields, Agr. 
Res. Rep. 64. 6, Pudoc. Wagcningen, 1958. 
-327-



Australian Grasslands,

Computation of the absolute maximum food production of the world. A report by the

Evaluation of models. In Ecological models, with emphasis on grassland models.

Transpiration and crop yields,

Van - Simulation of w~ter use and herbage growth in arid regions. Simulation Monographs, Pudoc, Wageningen (published : spring

World Survey of Climatology :

Name not available

Potential primary production of unirrigated land in the Sahel

Abstract

Similar works

Full text

Available Versions

NARCIS

Wageningen Yield

Name not available