Water Resources in the Desert Southwest - Design with the Desert: Conservation and Sustainable Development - page 77

Xerophytic
grassland

Cottonwood Sacaton

grassland

Mesquite
bosque

A. Initial
conditions
(<1880)

Groundwater
table

Dying
cottonwood

Sacaton
grassland

Mesquite
bosque

B. Initial
downcutting
(1880-1920)

Groundwater
table

Xerophytic
shrubs/grasses

Mesquite
bosque

C. Widening
(~1920-1940)

Barren

Groundwater
table

Xerophytic
shrubs/grasses

Mesquite
bosque

Riparian
woodland

D. Narrowing
(1940-present)

Groundwater
table

FIGURE 4.3
Schematic diagram showing the interaction between arroyo downcutting and riparian vegetation in southern
Arizona.

4.5 Surface Water
Several metrics describe flow in rivers. Flow is segregated by water years, normally
defined from October 1 through September 30; because the traditional definition splits the
critical early fall tropical-cyclones season, we redefine the water year in the Southwest as
November 1 through October 31. 9 Discharges are calculated using stage-discharge rela-
tions, developed from a combination of streamflow measurements and indirect-discharge
calculations following floods.22 22 Stage, the height of water above an arbitrary datum, is the
only continuously measured variable at gauging stations, usually recorded at 15 min inter-
vals. Over a 24 h period, unit discharges are averaged to create daily discharge, the basic
measurement of surface-water resources.