Journal
Journal of Hydrology 173 (1995) 41-50
A conceptual model of catchment water balance: 2.
Application to runoff and baseflow modeling
V.M. Poncea’*, A.V. Shettyb
“Department of Civil Engineering, San Diego State University, San Diego, CA 92182, USA
bNarional Institute of Hydrology, Roorkee, Uttar Pradesh, 247 667, India
Received 22 February 1994; revision accepted 17 February 1995
Abstract
A conceptual model of catchment water balance developed in the companion paper (Ponce
and Shetty, J. Hydrol., 173: 27-40, 1995) is used to simulate changes in runoff and baseflow
with annual precipitation. The model is based on the sequential separation of annual precipita-
tion into surface runoff and wetting, and wetting into baseflow and vaporization. Runoff is the
sum of surface runoff and baseflow. Runoff gain is defined as the derivative of runoff coefficient
with respect to precipitation. Baseflow gain is defined as the derivative of baseflow coefficient
with respect to precipitation. Catchment data show that runoff and baseflow gains are always
positive. Runoff gain reaches a peak value at a threshold precipitation P,,; baseflow gain
reaches a peak value at a threshold precipitation P,,. Analysis of the runoff and baseflow
functions sheds additional light on the nature of the competition between runoff and vaporiza-
tion, and baseflow and vaporization.
1. Introduction
The companion paper (Ponce and Shetty, 1995) has described a conceptual model
of water balance which separates annual precipitation into surface runoff and wet-
ting, and wetting into baseflow and vaporization, following concepts suggested by
L’vovich (1979). The objective is to determine the fraction of annual precipitation
which goes into surface runoff and the fraction of wetting which goes into baseflow.
Runoff, or streamflow, consists of surface runoff and baseflow. A competition exists
between surface runoff and wetting. Any drop of water that goes into surface runoff is
a drop that does not add to catchment wetting. Similarly, a competition exists
* Corresponding author
0022-1694/95/$09.50 0 1995 - Elsevier Science B.V. All rights reserved
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42 V.M. Ponce, A.V. Shetty / Journal of Hydrology 173 (1995) 41-50
between baseflow and vaporization. Any drop of water that goes into baseflow is a
drop that does not leave the catchment through vaporization.
Conflicting evidence regarding this dual competition is scattered throughout the
literature. On a global basis, differences in biogeographical regions and climatic
settings have tended to mask the nature of the competition. For instance, Pitman
(1978) has documented declining streamflow following large-scale afforestation in
South Africa. Baker (1986) has reported temporary increases in streamflow following
herbicide treatment in a pinyon-juniper watershed in north-central Arizona.
Ruprecht and Schofield (1989) have documented increases in both surface runoff
and baseflow following replacement of deep-rooted native forest species with shal-
low-rooted agricultural species in Western Australia. However, Ruprecht and Stone-
man (1993) have stated that the long-term prognosis for annual water yield from
areas subjected to forest harvesting in Western Australia is uncertain. Hibbert
(1967) qualified his review of catchment experiments to increase water yield by stat-
ing that catchment response to treatment was highly variable, and for the most part,
unpredictable. Nevertheless, Bosch and Hewlett (1982), in their comprehensive review
of catchment experiments to increase water yield, were not inclined to support Hib-
bert’s doubts. The issue of the competition between runoff (including baseflow) and
vaporization is one of the significant practical interest, as decreasing runoff is invari-
ably associated with decreasing water resources.
In this paper we simulate runoff and baseflow using a conceptual model of water
balance described in the companion paper (Ponce and Shetty, 1995). The conceptual
basis of the model enhances its applicability to a wide range of biogeographical
regions and climatic settings. In this paper, selected catchment data for Africa,
North America, and South America (L’vovich, 1979) are supplemented with the
writers’ own Asian data. Two runoff and baseflow functions are derived: (1) runoff
and baseflow coefficients vs. annual precipitation; (2) runoff and baseflow gains vs.
annual precipitation. Analysis of these functions leads to a characterization of runoff
and baseflow throughout the climatic spectrum, shedding additional light on the
competition between runoff and vaporization, and baseflow and vaporization.
2. The conceptual model
Annual precipitation P can be separated into two components (Ponce and Shetty,
1995):
P=S+W (1)
in which S is surface runoff and W is wetting. In turn, wetting is separated into two
components:
w=u+v (2)
in which U is baseflow, and V is vaporization. As used here, the term ‘vaporization’
encompasses all moisture returned to the atmosphere by evaporation: evapotran-
spiration from vegetated areas, evaporation from nonvegetated areas, and evapora-
tion from water bodies.