Observing Seasonal and Diurnal Hydrometeorological Variability within a
Tropical Alpine Valley: Implications for Evapotranspiration
GC41A-0108
Dr. Rob Hellström, Geography, Bridgewater State College and Dr. Bryan G. Mark, Geography, The Ohio State University
Department of Geography, Bridgewater State College, 24 Park Ave., Bridgewater, MA 02325, USA [rhellstrom@bridgew.edu] 1-508-531-2842
Department of Geography, The Ohio State University, 154 N Oval Mall, Columbus, OH 43210, USA [mark.9@osu.edu] 1-614-247-6180
Study Area (Llanganuco Valley, Cordillera Blanca, Peru)
Abstract
Conditions of glacier recession in the seasonally dry tropical Peruvian Andes motivate research to better constrain the hydrological balance
in alpine valleys. There is an outstanding need to better understand the impact of the pronounced tropical hygric seasonality on energy and
water budgets within pro-glacial valleys that channel glacier runoff to stream flow. This paper presents a novel embedded network installed
in the glacierized Llanganuco valley of the Cordillera Blanca (9ºS) comprising eight low-cost, discrete temperature and humidity
microloggers ranging from 3470 to 4740 masl and an automatic weather station at 3850 masl. Data are aggregated into distinct dry and wet
periods sampled from two full annual cycles (2005-2007) to explore patterns of diurnal and seasonal variability. The magnitude of diurnal
solar radiation varies little within the valley between the dry and wet periods, while wet season near-surface air temperatures are cooler.
Seasonally characteristic diurnal fluctuations in lapse rate partially regulate convection and humidity. Steep lapse rates during the wet
season afternoon promote up-slope convection of warm, moist air and nocturnal rainfall events. Standardized grass reference
evapotranspiration (ET0) was estimated using the FAO-56 algorithm of the United Nations Food and Agriculture Organization and will be
compared with estimates of actual ET from the process-based BROOK90 model that incorporates more realistic vegetation parameters.
Comparisons of composite diurnal cycles of ET for the wet and dry periods suggest about twice the daily ET0 during the dry period,
attributed primarily to the 500% higher vapor pressure deficit and 20% higher daily total solar irradiance. Conversely, the near absence of
rainfall during the dry season diminishes actual ET below that of the wet season by two orders of magnitude. Nearly cloud-free daylight
conditions are critical for ET during the wet season. We found significant variability of ET with elevation up through the valley. Humidity
and temperature measurements were analyzed to show significant effects of elevation and proximity to melt-water lakes on vapor pressure
deficit.
N
Navados
Huandoy
(6395 m a.s.l
a.s.l))
Navado
Huascarán
Huascar
án
(6768 m a.s.l
a.s.l))
Looking SW down valley:
Sensors deployed from base of lower lake to
viewpoint and on right (north) wall
10 km
Embedded Lascar Ta/RH Loggers
(elvations in masl)
Data & Methods
4561
4544
4355
4775
3955
4122
3846
Background & Questions
• The Cordillera Blanca is seasonally isothermal
• The seasonal migration of the ITCZ creates distinct dry (MaySept.) and wet (Oct.-April) seasons
• The magnitudes of regional latent heat flux and
evapotranspiration are unverified in tropical alpine regions
• How do local atmospheric conditions (wind vectors and lapse
rate) within Tropical alpine valleys alter seasonal surface
moisture fluxes: evapotranspiration (ET)?
• Driving questions:
–Does microscale variability of temperature, humidity and
topography significantly modulate catchment hydrology?
–Can we develop a sustainable, low-cost embedded sensor
network and apply FAO-56 or ASCE Penman-Monteith
algorithms (12 cm reference grass) of the Ref-ET model (Allen,
et al., 1998) to evaluate elevation and aspect impacts on
moisture flux in an alpine valley?
Lower Lake (3833 masl)
Dry Period Ta & RH Wet Period
3833
3458
North Wall (4544 masl)
Dry Period Ta & RH Wet Period
Portachuela (4775 masl)
Dry Period Ta & RH Wet Period
• Sources of ground-based data within valley
– 4 HOBO Microstations, Automatic Weather Station
– 9 Lascar air temperature and humidity sensors embedded at
different elevations (with solar shield)
• ET-REF and BROOK90 models
– Estimating potential reference evapotranspiration
• Compare diurnal variations in meteorological forcing for dry
and wet seasons, each a 44-day period based on historical
precipitation records
– Dry: 19 July => 31 August 2006
– Wet: 1 January 2006 => 13 February 2007
Lascar
Lascar + Microstation
Conclusions
Solar & Precip.
Solar & Precip.
Solar & Precip.
Vapor Pressure & VP Deficit.
Vapor Pressure & VP Deficit.
Wind Speed
Wind Speed
ET0 ASCE & FAO-56 P-M
ET0 ASCE & FAO-56 P-M
Air Temperature Profiles
Dry Period
Wet Period
Vapor Pressure & VP Deficit.
Vapor Pressure Deficit Profiles
Dry Period
Wet Period
Wind Speed
• The near surface average daily lapse rate was 6.91 °C/km for
the dry and 6.09 °C/km for the wet periods, which
extrapolated the 0°C isotherm to 5135 masl for the dry and
5375 masl for the wet periods.
• The vapor pressure deficit decreases more rapidly during the
dry period, but extrapolates to zero at a lower elevation during
the wet period
• The heterogeneity of the alpine valley creates microclimate
zones that depend largely on elevation and surface orientation,
which modulates ET potential
• Most precipitation (and cloud cover) occurs between sunset
and sunrise during the wet season, hence insolation at the
ground is strong during both seasons
• Valley winds dominate during sunlight hours in both seasons,
but are more dominant during the wet period, and abruptly
shift to katabatic winds after sunset during the dry season
• The combination of valley winds and steep lapse rates between
1200 and 1500 during both seasons suggest potential for local
warm air advection contributions to ET and glacial melt
• Furthermore, the up-slope winds may enhance convective
precipitation, particularly during the wet season after sunset
• The vapor pressure deficit decrdeases
• We need field measurements of vegetation properties in the
Llanganuco Valley to obtain an empirical crop coefficient to
adjust ET0 and better estimate actual ETC
Acknowledgements
Wind Speed vs. Direction
Dry Period
Wet Period
Wet Period Wind Speed versus Direction
Dry Period Wind Speed versus Direction
3.5
3.5
Valley Orientation
55° => 235°
Valley Orientation
55° => 235°
3
Wind Speed (m/s)
Wind Speed (m/s)
3
ET0 ASCE & FAO-56 P-M
2.5
2
1.5
1
2.5
2
1.5
1
0.5
0.5
0
0
0
45
90
135
180
225
Wind Direction (°)
270
315
360
0
45
90
135
180
225
Wind Direction (°)
270
315
360
Funding for this project was provided by The Ohio State University,
Department of Geography and Office of International Affairs, and the
Presidential Fellowship at Bridgewater State College. We are grateful
for the collaboration with Peruvian Institute of Natural Resources
(INRENA), especially the data and logistical support by Jesús Gómez,
INRENA-Huaraz, Perú and OSU graduate student assistant Karin
Bumbaco for help with data collection summer 2007.
Model Reference
Allen, R.G., Pereira, L.S., Raes, D., and Smith, M. 1998. Crop
evapotranspiration-Guidelines for computing crop water
requirements, FAO irrigation and drainage paper 56, FAO. ISBN
92-5-104219-5.