otherwise be producing anything. The shadow wage would be the wage that
the worker could get in the next best occupation. If that alternative occupa-
tion does not exist, then the shadow wage approaches zero. The idea here is
that employing someone has a positive social benefit and that projects with
this effect have larger social profits than first appears to be the case. A second
illustration relates to foreign trade. If crops are not grown indigenously, they
would have to be imported. Similarly, any crop grown and consumed domes-
tically forgoes export revenues of the crop that could have been exported. The
relevant shadow price therefore becomes the price that the crop would have
gotten if it were internationally traded, its so-called border price.
Barbier’s analysis is conducted both for conventional financial costs and
revenues and also in terms of shadow prices. Looking at the financial analysis
first, we see that, on average, all crops other than sesame actually make finan-
cial losses, whereas A. senegalmakes a profit. Second, mixed farming systems
as a whole make profits in only three regions: Blue Nile, North Kordofan, and
South Kordofan. In contrast, A. senegalis profitable in all regions. Sesame is
also seen to be very profitable in the regions where there is evidence of farm
income. This raises the issue of why multiple crops, including gum trees, are
grown in regions where sesame is profitable. There appear to be two possible
answers. First, mixed outputs are a risk aversion strategy: events adversely
affecting one crop may not affect others. Second, the crops serve both a mar-
ket strategy and subsistence strategy. Sorghum and millet are grown primarily
for subsistence. Third, there are interdependencies between the crops and gum
arabic. These interdependencies reflect the environmental benefits of gum ara-
bic, that is, the returns to cropping would not be what they are but for the
external benefits of gum arabic. But they also reflect the fact that gum arabic
from A. senegalcan be harvested at different times of the year than crops, so- The Economic Valuation of Agroforestry’s Environmental Services 77
Table 4.1. Economic returns to mixed Acacia senegaland crop production
in the Sudan.a
Area A. senegalb Sorghum Millet Groundnut Sesame Total
Blue Nile 334 589 233 — — 1,156
White Nile 259 –1,507 –1,088 –231 20 –2,547
North Kordofan 56 — –117 — 307 246
South Kordofan 81 –586 — 601 1,375 1,471
North Darfur 87 — –408 –910 — –1,231
South Darfur 95 –1,180 –404 –726 — –2,215
All systems average 152 –671 –357 –316 567 –520
Average at shadow pricesc 2,065 2,412 2,330 9,461 9,468 16,656
Source:Barbier (1992).
aNet present value in Sudanese pounds per feddan = 0.4 ha.
bGum fodder and fuelwood.
cSee text for explanation of shadow prices.