I. HYDROGEOLOGY OF ARID REGIONS
11
map and receives an inflow of water in the eastern edge. Using Eq. (5)
and the flow net, together with the scale, the southern stream is calculated
to be recharging the groundwater with 664 m
3
/day/km at A, 0.0
m
3
/day/km at B, and a discharge of 592 mVday/km to the stream at
C.
These figures are calculated as follows :
Qi = Q 2
since both Ah and T are constant in the area of the map.
Q 1 = Q 2 = Ah(T) = (0.5) meters (650) m
2
/day = 325 m
3
/day
the length of stream between flowlines at A = 0.98 km, and the length
of stream between flowlines at C = 1.10 km, therefore the outflow at
A = 325/0.98 = 332 mVday/km for one side of the stream, or a total
outflow of 664 m
8
/day/km; likewise at C the inflow is 592 mVday/km.
Groundwater flow at B is parallel with the stream channel, hence water
is neither entering or leaving the stream channel.
Flow nets can also be used to represent flow in a vertical plane. In fact,
one of the most common applications of flow nets is in the analyses of
seepage through dams within vertical sections (Davis and De Weist,
1966). The general water-table profile shown in Fig. 1 is commonly observed in desert mountains and valleys. Using flow-net sketching, one can
demonstrate two reasonable configurations of flow that will explain the
strong inflection in the water surface near the base of the mountains. One
explanation assumes a change in Q owing to spring discharge. If large
springs do not exist, then the other geologically reasonable assumption of
a drastic change in hydraulic conductivity, K is favored. The vertical flow
net also points out something of widespread importance concerning hydraulic heads, namely, wells will encounter decreasing heads with depth
in areas of recharge and increasing heads with depth in areas of discharge.
IV. Groundwater Recharge
Recharge of groundwater is an extremely rare event in deserts except
under very special hydrogeologic conditions (Dixey, 1962). Most precipitation, whether it totals 1 or 10 mm is returned to the atmosphere by
evaporation within 2 or 3 days after the water reaches the desert surface.
During periods of extremely heavy precipitation, soils may absorb a large
fraction of the water. Capillary action will bring this water back to the
soil surface where it is evaporated. Some of the soil water may also be
11
map and receives an inflow of water in the eastern edge. Using Eq. (5)
and the flow net, together with the scale, the southern stream is calculated
to be recharging the groundwater with 664 m
3
/day/km at A, 0.0
m
3
/day/km at B, and a discharge of 592 mVday/km to the stream at
C.
These figures are calculated as follows :
Qi = Q 2
since both Ah and T are constant in the area of the map.
Q 1 = Q 2 = Ah(T) = (0.5) meters (650) m
2
/day = 325 m
3
/day
the length of stream between flowlines at A = 0.98 km, and the length
of stream between flowlines at C = 1.10 km, therefore the outflow at
A = 325/0.98 = 332 mVday/km for one side of the stream, or a total
outflow of 664 m
8
/day/km; likewise at C the inflow is 592 mVday/km.
Groundwater flow at B is parallel with the stream channel, hence water
is neither entering or leaving the stream channel.
Flow nets can also be used to represent flow in a vertical plane. In fact,
one of the most common applications of flow nets is in the analyses of
seepage through dams within vertical sections (Davis and De Weist,
1966). The general water-table profile shown in Fig. 1 is commonly observed in desert mountains and valleys. Using flow-net sketching, one can
demonstrate two reasonable configurations of flow that will explain the
strong inflection in the water surface near the base of the mountains. One
explanation assumes a change in Q owing to spring discharge. If large
springs do not exist, then the other geologically reasonable assumption of
a drastic change in hydraulic conductivity, K is favored. The vertical flow
net also points out something of widespread importance concerning hydraulic heads, namely, wells will encounter decreasing heads with depth
in areas of recharge and increasing heads with depth in areas of discharge.
IV. Groundwater Recharge
Recharge of groundwater is an extremely rare event in deserts except
under very special hydrogeologic conditions (Dixey, 1962). Most precipitation, whether it totals 1 or 10 mm is returned to the atmosphere by
evaporation within 2 or 3 days after the water reaches the desert surface.
During periods of extremely heavy precipitation, soils may absorb a large
fraction of the water. Capillary action will bring this water back to the
soil surface where it is evaporated. Some of the soil water may also be