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(a)
Liquid Water (mm)
Dry Period: Modeled Hydrological Components
Precip. Soil Evap. Transpiration Intercepted Evap. Infiltration
1
0.8
0.6
0.4
0.2
0
-0.2
0:00
2:00
4:00
6:00
Figure 16. BROOK90 modeled components of ET for composite diurnal cycle for dry (a) and wet
(b) periods.
DISCUSSION
Time of Day (hour)
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Field and model results were interpreted and compared to explain the processes controlling ET
within a pro-glacial valley.
Meteorological Measurements
The new sensor network initiated by this project permitted high resolution, discrete monitoring
of air temperature at different elevations within a pro-glacial valley. Calibration of iButton
temperature logger with the shielded temperature sensor on the HOBO weather station suggested a
strong correlation, r 2 =0.988, and this gave us high confidence in our interpretation of the
measurements. As expected, analysis of hourly data from 2005 revealed evidence for a strong
diurnal and wet versus dry seasonal dependence of meteorological forcing within the valley. The
dry/wet ratio of 1.27 for daily insolation is smaller than we expected given the absence of
precipitation during the dry period and much high precipitation total for the wet period. The
hourly composite of precipitation for the wet period (Fig. 9) clearly shows the nocturnal tendency
for precipitation and hence convective cloud formation at night, and strong solar forcing during
daylight hours.
However, solar forcing was surprisingly similar for both the dry and wet periods, since most
convection (rainfall) during the wet season occurs an hour or two prior to sunset and dissipates
prior to sunrise. This strong diurnal convective cycle coupled with a persistent up-valley wind and
a strong daytime lapse rate during the wet season suggests the plausible influence of daytime
surface heating west and down slope of the pro-glacial valley. Furthermore, since clouds
accompany precipitation, we concluded that nocturnal cloud cover strongly influences
interseasonal and diurnal cycles of net radiation. Furthermore, this may establish a connection
between cloud cover (and precipitation) and anthropogenic development in the Santa River valley,
which receives most of its water from pro-glacial valleys along the western Cordillera Blanca,
such as Llanganuco (Fig. 1). As demonstrated by Vuille et al. (2003), it is the western slope of the
Cordillera Blanca that is experiencing the highest rate of warming based on weather station
records. If this warming trend continues, we would expect to find drier and stronger up valley
winds during both seasons. Forced by orographic uplift of the western slope, these near-surface
winds would promote stronger nocturnal convection during the wet season.
This is one of our arguments for continued expansion of ground-based meteorological networks
within tropical proglacial valleys. We are also interested in more accurately estimating the
evapotranspiration rate within the valleys. Our model results are preliminary and we will require
additional field measurements to verify and more accurately initiate the BROOK90 model.
Liquid Water (mm)
275
Wet Period: Modeled Hydrological Components
Precip.
1
Soil Evap. Transpiration Intercepted Evap. Infiltration
0.8
0.6
0.4
0.2
0
-0.2
0:00
2:00
4:00
6:00
Time of Day (hour)
8:00
10:00
12:00
14:00
16:00
18:00
20:00
22:00