Spatial patterns of interaction among climate variability and change, soil water deficit and transpiration
in small mountain catchments of Southern Sierra Critical Zone Observatory
Kyongho Son and Christina Tague, Bren School of Environmental Science and management ,
University of California, Santa Barbara ( kson@bren.ucsb.edu)
Abstract
Effect of climate warming on hydrologic response of the CZO watersheds
Hydrologic Similarity index
Snow
Soil
moisture
Refining Processes
Transpiration
(Penman-Monteith)
Transpiration
Clustering analysis (PAM algorithm)
Constraining parameters
Photosynthesis (Farquhar)
P303
Horizontal hydrologic processes
B203
Stomatal Conductance Javis Model
Gs=f(Tmax,Tmin,LWP, CO2, Radiation, VPD)
Gs.canopy=gs*LAI
LWP-related to soil water availability
Linked with distributed hydrologic model
and its parameters
(Tague,2010)
Conceptual model for effect of climate warming on ecosystem water use (tague, et al. 2009)
Spatial and temporal variability of collected soil moisture and sapflux data
Daily mean soil moisture and sapflux data of sampling sites.
Hydrologic Similarity indices
mean and inter-annual variation (expressed
as coefficient of variation, CV) of five
indicators
(a) number of days of snow melt
(b) day of water year that root-zone soil
moisture is fully saturated
(c) day of water year that root-zone soil
moisture declines to 70% of saturation
(d) day of water that root-zone soil moisture
declines to 50% of saturation
(e) day of water year that transpiration
declines to 50% of its peak growing season
value
(f) Combined five indices
Generated Spatial and temporal values
of snow, soil moisture and transpiration
Carbon and nitrogen processes
Conceptual model for Improving ecohydrologic predictions
using collected data
CZT4
CZT7
CZT5
CZT8
CZT6
CZT3
Snow-dominated watershed (B203)
Snow, Soil moisture,
Transpiration,
ET and Streamflow
Vertical hydrologic processes
Locations of selected sampling sites, and flow drainage
areas for each sampling site
Conceptual framework of top-down approach for soil
moisture and transpiration data collection
Calibrated RHESSYs model
RHESSys Modelling Framework
Snow-rain transition watershed (P303)
In snow-dominated mountain systems, a warming climate alters soil water deficit through changing the timing and
magnitude of moisture inputs as precipitation and snowmelt and through changes in the timing and magnitude of
evapotranspiration losses. The net effect of climate warming on soil water deficit and associated ecosystem
processes ultimately depends on the interaction between changes in inputs and outputs and on vegetation, microclimate and soil properties that control the sensitivity of soil water to changes in input/output drivers. In mountain
environments, steep spatial gradients result in substantial variation in atmospheric forcing and vegetation and soil
properties over relatively short spatial scales, which necessitate providing finer-scale assessment of climate change
impact. Measurements of soil moisture and forest responses to climate are often made at plot scales but are
limited in spatial coverage. Coupled eco-hydrologic models, applied at relatively fine (m) scales provide a method of
extrapolating findings from local measurements and exploring hillslope and watershed scale impacts of climate
warming on moisture deficit. In this study, we use RHESSys (Regional hydro-ecologic simulation system) combined
with spatially intensive monitoring of coupled ecohydrologic variables at the Southern Sierra Critical Zone
Observatory (SSCZO), located in the Sierra National Forest, California. We initially use the model to identify clusters
of distinctive water deficit behavior as summarized by indices of summertime soil moisture and transpiration
recessions. The resulting clusters demonstrate that both elevational differences in energy availability and
topographic controls on drainage are 1st order controls on spatial patterns of summertime moisture deficit and
tree transpiration. We then use these initial model clusters to guide soil moisture and sapflux data collection. The
collected data are used to characterize soil moisture deficit and transpiration for each cluster evaluate and improve
the model predictions. Our initial results highlight the importance of adequate representation of micro-climate
patterns as controls on summer moisture deficits and transpiration. We then use the model to show how spatial
patterns of summertime moisture deficit and transpiration may change under future climate scenarios. We apply
the approaches which are using 2 and 4 C° uniform temperature adding to the historic meteorological data.
Spatial pattern of Monthly ET
Climate warming effect on annual ET (mm)
Finding Clusters and Medoids
Sampling
Recalibrated model
Clustering analysis results of five hydrologic similarity indices
Model Calibration results : (left) snow and (right) streamflow
The measured soil moisture and sapflux showed strong seasonal pattern, while each site showed different temporal patterns.
CZT5, CZT6 and CZT7 have higher soil moisture and transpiration than the other sites. CZT7 site has large upslope drainage areas and thus maintains higher soil
moisture and transpiration in the summer period than other sites. However, CZT6 also has high summer soil moisture and transpiration, but the site has
relatively small drainage area based on surface topography. Thus, surface topography does not always capture the pattern of subsurface flow. In winter, some
sites can maintain high transpiration but other sites are more temperature limited. Key question is whether the model can sufficiently resolve these
microclimate patterns.
P303
Change in
Annual ET
P303
B203
T2-T(base)
-5.5
24
T4-T(base)
-13
54
B203
Low ET area
Spatial variability of microclimate, soil moisture and transpiration
1) Dry site
(Cluster 1)
2) Intermediate site
(Cluster 3)
3) Wet site
(Cluster 5)
Snow-dominated watershed(B203) ET increases early
growing season and decreases later in summer due to water
stress.
Net effect varies spatially. Average snow-rain transition
watershed(P303) ET changes are much smaller.
The spatial pattern of May ET in P303 is similar to the flow
drainage pattern. In upslope areas, warming reduces ET, and
in wetter riparian areas, warming increases ET. In B203,
warming causes the greatest ET reduction occurs in the
lowest elevation areas.
Final clustering results and the location of data collections
Low Elevation
The relationship among microclimate (VPD),
soil moisture and transpiration in the
summer varied between sites.
Five hydrologic similarity indices are calculated based on the model estimates
of snow, soil moisture and transpiration (Fig.1). The cluster of (a) index represents the
elevation difference of watershed (relating to the energy distribution). The
clusters of (b), (c) and (d) represent the topography controls on flow drainage
patterns. Combining all indices represent two patterns, which are elevation
difference and topography controls on the flow drainage.
Based on the clustering results, we decide six representative plots.
Each plot has five soil pits and a white fir tree. We measured soil moisture and
temperature and electric conductivity with 10 5TE sensors and measured the sapflux of
white fir tree with the heat-pulse type sensor
In the dry sites (CZT3), Transpiration is
correlated with soil moisture and limited
by available soil moisture. In the
intermediate site (CZT6), the transpiration
is highly correlated with VPD.
However, in the wet site (CZT7), the
transpiration is not correlation with soil
moisture and VPD. We expect that the
temporal pattern of transpiration at the
wet site is correlated with receiving
radiation.
Preliminary Model Simulation Results at Plot Scale
• Plot-scale(30m) modelling at CZT3(Cluster 1) using randomly 100-sampled soil parameters (m, Ksat, pore
size index, air entry pressure) to test the sensitivity of soil moisture and transpiration to soil parameter
uncertainty.
Summary
o Estimates of hydrologic similarity indices using a physical distributed model are used to
strategically guide site selection for soil moisture and sapflow measurements.
o Collected soil moisture and sapflux data suggest that microclimate is significant controls on
summer and winter transpiration.
o Snow-dominated watershed (B203) is more sensitive to climate warming than snow-rain
transition watershed (P303)
o Warming can both increase PET and decrease available water by shifting the timing of
water input (snow vs rain). The net effect on ET varies spatially with upslope areas in the rainsnow transition watershed showing greater reduction of ET and lower elevation in the snowdominated watershed.
This project is funded by grants from the National Science Foundation-California Sierra Critical
Zone Observatory and Kearney Foundation of Soil Science