dry season. The flashy response of the Mae Sa Noi sub-basin was also very
pronounced, as shown by the flow duration curves depicted in Fig. 3.4.
Figure 3.4 shows that while for the Mae Sa and Headwater station the doublelogarithmic plots of mean specific discharge versus percentage of time followed a
similar line, for the Mae Sa Noi sub-basin the slope was much steeper, in the range
below Q 50 , indicating a more uneven discharge distribution during high-flow
conditions. In contrast, discharge at the Mae Sa Noi flume showed nearly no
variation during low-flow conditions (discharge < Q 50 ). During these periods
each year, the stream was fed by a low but continuous groundwater flow.
Figure 3.5 shows the relationship between stream discharge (Q) and its derivative with respect to time (dQ/dt) for the three gauging stations. In the idealized case
that a watershed consists of only a single reservoir which empties in a linear
manner, the relation between both variables can be described by the following
ordinary differential equation:
dQ
dt
¼ ÀaQ
b
(3.1)
Here, b (1) equals unity and a (1/d) is the recession coefficient. Such an idealized
situation can be described in the easiest way using the picture of a bucket filled with
water. Imagine that the bucket is equipped with a valve at its bottom. If one opens
the valve, at the very beginning water will flow very quickly out of the bucket;
however, as time passes, the flow of water will continuously decline until the
reservoir is empty. Under such simple conditions, plotting a log (ÀdQ/dt) versus
log Q results in a straight line with a slope of unity (b ¼ 1). Thus, the more you
open the valve, the more quickly the water will drain out of the bucket. This effect is
taken into account in (3.1) by the parameter a. The greater the value of a, the faster
Fig. 3.4 Flow duration curves for the gauging stations within the Mae Sa watershed. Curves were
derived from discharge data for the years 2007–2010
3 Water and Matter Flows in Mountainous Watersheds of Southeast Asia:. . .
119
pronounced, as shown by the flow duration curves depicted in Fig. 3.4.
Figure 3.4 shows that while for the Mae Sa and Headwater station the doublelogarithmic plots of mean specific discharge versus percentage of time followed a
similar line, for the Mae Sa Noi sub-basin the slope was much steeper, in the range
below Q 50 , indicating a more uneven discharge distribution during high-flow
conditions. In contrast, discharge at the Mae Sa Noi flume showed nearly no
variation during low-flow conditions (discharge < Q 50 ). During these periods
each year, the stream was fed by a low but continuous groundwater flow.
Figure 3.5 shows the relationship between stream discharge (Q) and its derivative with respect to time (dQ/dt) for the three gauging stations. In the idealized case
that a watershed consists of only a single reservoir which empties in a linear
manner, the relation between both variables can be described by the following
ordinary differential equation:
dQ
dt
¼ ÀaQ
b
(3.1)
Here, b (1) equals unity and a (1/d) is the recession coefficient. Such an idealized
situation can be described in the easiest way using the picture of a bucket filled with
water. Imagine that the bucket is equipped with a valve at its bottom. If one opens
the valve, at the very beginning water will flow very quickly out of the bucket;
however, as time passes, the flow of water will continuously decline until the
reservoir is empty. Under such simple conditions, plotting a log (ÀdQ/dt) versus
log Q results in a straight line with a slope of unity (b ¼ 1). Thus, the more you
open the valve, the more quickly the water will drain out of the bucket. This effect is
taken into account in (3.1) by the parameter a. The greater the value of a, the faster
Fig. 3.4 Flow duration curves for the gauging stations within the Mae Sa watershed. Curves were
derived from discharge data for the years 2007–2010
3 Water and Matter Flows in Mountainous Watersheds of Southeast Asia:. . .
119