16
STANLEY N. DAVIS
flows. Most commonly, however, water seeps to the surface over relatively
large areas where it may be intercepted by vegetation before it reaches
the surface.
The term phreatophyte was introduced by Meinzer (1927) to describe
plant types that grow on dry land but habitually send their roots into the
upper part of the zone which is saturated with water. Although the term
has not gained wide acceptance among biologists, it is useful in considering
problems of desert hydrology because it emphasizes the fact that some
plants draw water directly from the groundwater reservoir while other
types of plants, generally called xerophytes, utilize only water from infrequent precipitation.
Phreatophytes give some information concerning the general depth of
the saturated zone, the quality of the water, and the total amount of
groundwater which is lost by évapotranspiration. In general, grasses which
are phreatophytes thrive where the water table is at a depth of less than
3 meters; shrubs thrive where it is at a depth of less than 10 meters; and
trees grow where the depth is less than 30 meters. Exact depths are highly
variable and depend on factors such as water salinity, plant species, and
types of sediments or rocks penetrated by the plant roots.
Some plants are reliable indicators of water quality. Willows and cottonwood generally grow where potable groundwater is present. Pickleweed
(Allenrolfea occidentalisa grows where the subsoil is saturated with saline
water. Other plants such as palms and mesquite have a tolerance for a
wide range of water quality.
The amount of water used by phreatophytes is a function of climate,
species, general health of the plant, depth of the water, water quality, and
density of growth. Estimates of water used by phreatophytes utilize direct
measurements of évapotranspiration coupled with estimates of vegetation
density (McDonald et ai, 1958). Plants such as saltcedar (Tamarix gallica) and cottonwood (Populus) will use between 2000 and 3000 mm of
water per year for dense growths in a hot, dry climate (Robinson, 1958).
Even though exact measurements of total water discharged by phreatophytes are difficult, rough estimates are easily made and are useful despite errors which may range up to 100% (Mann, 1958).
The quantity of water used by phreatophytes is truly spectacular. For
example, a dense growth of saltcedar over an area of only 1 hectare (2.47
acres) will use at least 20,000 m
3
of water each year. This is 30 million
liters, or 54,800 liters/day for an entire year, or enough to give a small
village an abundant supply of water. In fact, as a general rule, people in
cities use less water per unit area than dense growths of phreatophytes.
Groundwater is commonly evaporated directly from the soil surface. If
the evaporation proceeds without rainfall, a salt crust will form. The salt
STANLEY N. DAVIS
flows. Most commonly, however, water seeps to the surface over relatively
large areas where it may be intercepted by vegetation before it reaches
the surface.
The term phreatophyte was introduced by Meinzer (1927) to describe
plant types that grow on dry land but habitually send their roots into the
upper part of the zone which is saturated with water. Although the term
has not gained wide acceptance among biologists, it is useful in considering
problems of desert hydrology because it emphasizes the fact that some
plants draw water directly from the groundwater reservoir while other
types of plants, generally called xerophytes, utilize only water from infrequent precipitation.
Phreatophytes give some information concerning the general depth of
the saturated zone, the quality of the water, and the total amount of
groundwater which is lost by évapotranspiration. In general, grasses which
are phreatophytes thrive where the water table is at a depth of less than
3 meters; shrubs thrive where it is at a depth of less than 10 meters; and
trees grow where the depth is less than 30 meters. Exact depths are highly
variable and depend on factors such as water salinity, plant species, and
types of sediments or rocks penetrated by the plant roots.
Some plants are reliable indicators of water quality. Willows and cottonwood generally grow where potable groundwater is present. Pickleweed
(Allenrolfea occidentalisa grows where the subsoil is saturated with saline
water. Other plants such as palms and mesquite have a tolerance for a
wide range of water quality.
The amount of water used by phreatophytes is a function of climate,
species, general health of the plant, depth of the water, water quality, and
density of growth. Estimates of water used by phreatophytes utilize direct
measurements of évapotranspiration coupled with estimates of vegetation
density (McDonald et ai, 1958). Plants such as saltcedar (Tamarix gallica) and cottonwood (Populus) will use between 2000 and 3000 mm of
water per year for dense growths in a hot, dry climate (Robinson, 1958).
Even though exact measurements of total water discharged by phreatophytes are difficult, rough estimates are easily made and are useful despite errors which may range up to 100% (Mann, 1958).
The quantity of water used by phreatophytes is truly spectacular. For
example, a dense growth of saltcedar over an area of only 1 hectare (2.47
acres) will use at least 20,000 m
3
of water each year. This is 30 million
liters, or 54,800 liters/day for an entire year, or enough to give a small
village an abundant supply of water. In fact, as a general rule, people in
cities use less water per unit area than dense growths of phreatophytes.
Groundwater is commonly evaporated directly from the soil surface. If
the evaporation proceeds without rainfall, a salt crust will form. The salt