PUB in practice: case studies - Runoff Prediction in Ungauged Basins: Synthesis across Processes, Places and Scales - page 320

Figure 11.53. Illustration of the soft
hydrological monitoring devices
used as (a) a stage recorder and
(b) a rain gauge. From Crabit et al.
( 2011a ).

22 cm

Waterproof container

USB connector Calibrated funnel

USB connector

Capacitor

Plastic
Tubes

Rigid plastic tube

14 cm

Hole

Water

Water

Level zero

evacuation
Soil

Soil

a) b)

Method
Framework to define the hydrological signatures
We propose a three-step framework to define hydrological
signatures at both the event and annual scales.

Hence, a range of values of roughness coefficients
were considered, resulting in a range of estimated
discharge values at each time step. Finally, the min-
imum and the maximum values of runoff were calcu-
lated for each flood event using the envelope of the
rating curves.
(3) Hydrological signatures: Among the large number of
possible indicators calculated from rainfall

(1) Soft hydrological monitoring: The specifications for
the measurement devices were that they are easy to
install and easy to use, non-perturbing, robust and able
to acquire high temporal frequency data with low
energy consumption. Experimental settings were
designed to acquire rainfall intensities and stage
records at catchment outlets at 1-minute time steps
( Figure 11.53 ). The chosen data logger consisted of a
capacitor made up of two conductors (copper and
water) and an insulator (Teflon). The measured capaci-
tance depending on the surface of its conductors
allows for the measurement of water depth. Laboratory
tests show accurate results in dynamic conditions with
a battery life of 3 weeks (Crabit et al., 2011a ).
(2) Estimation of runoff: Discharges at catchment outlets
were estimated from the water depth measurements
using rating curves. The rating curves were
established for each outlet section, using the Manning
equation and by taking into account specific rough-
ness conditions. For each type of vegetation encoun-
tered in ephemeral streams (which is often non-
aquatic vegetation), roughness coefficients were
experimentally determined in controlled conditions
(Crabit et al., 2011b ). Since the depth gauge was
precisely measured, the discharge uncertainty came
mainly from the values of the roughness coefficients.

runoff
data, only those relevant for qualifying intermittent
flows in ephemeral streams were retained (Crabit
et al., 2011b ): the annual runoff coefficient A (denoted
A min and A max when using respectively the minimum
and maximum values of the estimated hydrograph), the
total number of rainfall events N (with a total daily
rainfall > 5 mm), the total number of events when
runoff occurs N r , and the frequency of the catchment
response B ¼ N r /N.

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Results
Hydrological signatures of the studied catchments
On the 12 studied catchments, the annual rainfall showed
slight spatial variations ranging between 352 and 548 mm/
yr ( Figure 11.54 ) according to a north

south gradient.
Even if the annual rainfalls were close for all the catch-
ments, the annual runoff coefficients were highly variable:
for example, A ranged between 0.01 and 0.02% for C7 and
between 13.0 and 24.5% for C2. Detailed analysis high-
lighted that one single flood event runoff could represent
50% of the annual runoff. For example, in catchment C2,
one rainfall event of 90.4 mm (representing 22% of the

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