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I am engaged
in research on hydroclimatology, the intersection of the science of
climatology with hydrology and hydrogeology. The goal is to develop a deep, fundamental understanding of the connections between climate and the terrestrial water cycle. An important result is the production of useful
quantitative metrics of the impacts of
climate change on water resources.
MotivationWhat
information do users and suppliers of water need to make wise
decisions? What needs to be done to provide the information that they
need? Water managers throughout the United States and the world are
making decisions today about large investments in new water facilities
for current and future needs that could take decades to complete. When
completed these efforts must solve water shortages not just for a few
years, but for many decades beyond.
Water supply
projects are planned using assessments of future water conditions which
have traditionally been estimated from past climate data. If an agency
needed to know the
future average yearly rainfall or how long the typical or worst drought
might last, then they examined the historical records for the last
hundred years and computed a value to use as a future estimate. However
climate change
research shows clearly that the future will not be like the past. In
fact, today is already not much like the past. Therefore incorporating
climate change projections into accurate forecasts of future available
water supply and demands is critical so that projects are not
immediately inadequate the moment they are completed. Past WorkMost past work has been research on
the impact of climate change on just winter mountain ice and snowpack.
Since about a third of California's water comes from this source, snowpack
is certainly important. However, parts of California and much of the
world rely primarily or even exclusively on water supply from other
sources like groundwater. These other sources are equally important and
will become even more heavily used and critical as snowpack
dwindles as a readily available supply. Therefore, it is necessary for
hydroclimatology research to investigate impacts on the full
water cycle and these other water sources.
In addition to considering precipitation received
in the form of snowfall, the amount of annual rainfall received
is essential to determine
both surface water and groundwater resources. But even with constant
precipitation, other climate aspects like predicted changes to
precipitation patterns such as more intense rain storms, shorter rain
season, etc could impact water resources. In addition, other aspects
like warming temperatures might increase losses from
evapotranspiration. So hydroclimatology research must also incorporate
the changes caused by all the various aspects of climate.
MethodologyMy
research analyzes how each water supply resource and element
of the hydrological cycle is impacted by a change of each climate
aspect. This is
much more complex than it might seem at first glance. Each climate
aspect can have a different relationship with and effect
on each
element
of the water cycle. It is
necessary to rigorously evaluate and quantify each of these
relationships and effects to better understand exactly how and why the
water system responds to climate changes.
Water Cycle Element Partitioning
My
research partitions climate impacts with a "follow the water" approach
to assess effects on successive elements of the terrestrial water
cycle, i.e. the path from precipitation, to surface features, to
sub-surface soil, then deep underground. Certainly the
fundamental source
of the water cycle is the precipitation deposited onto the surface as
rain,
sleet, hail, snow, or other water forms. As soon as
the precipitation falls then it immediately starts to partition off
into
different flows and stores.
A significant fraction of precipitation may
be subject to interception by
the vegetation canopy or pooling
at the
ground surface and then to loss from evaporation
or sublimation. The
precipitation not
diverted into surface capture is then subject to infiltration into the ground. Any
infiltration excess water can then
become storm runoff into
surface water bodies.
Infiltrated water wets and is absorbed by the soil from which it can evaporate away into the air even
from soil pores below the surface. ![[Evapotranspiration diagram]](Surface_water_cycle.jpg) Water in the soil root zone is subject to uptake
by vegetation and then to transpiration
through its leaf surfaces into the air. Throughflow or interflow is the lateral movement
of water in the vadose
zone, i.e. below the soil surface and above the groundwater
table. As
the water moves downwards under gravity and the soil
becomes more compact and less permeable with increasing depth, thus
some water
tends to move sideways. Percolation
is the slower downward movement of the rest of the sub-surface water
through the vadose zone.
Water that percolates downward eventually reaches the saturated water
table and acts to recharge
that ground water. ![[Vadose & baseflows]](Vadose_zone.gif) Shallow
groundwater
located near and above the levels of local streams, springs, wetlands,
lakes, and other surface water features may exfiltrate (flow out of the
ground) or discharge and
contribute to surface water bodies. Since half of the
water that percolates down to shallow ground water may contribute to long term baseflow of such streams, it can provide a possibly year round important source of surface
water supply. Adequate baseflow is also vital as a supply for
riparian vegetation, fish, amphibians and other water dependent
life.
Groundwater in coastal areas can become submarine
groundwater discharge (SGD) which is the net flow of fresh water
through an aquifer out into the sea. The reverse flow called saltwater intrusion is seawater
flowing inland into the freshwater
aquifers. Finally, the extraction
of groundwater from aquifers
via the pumping of wells or flow from springs can provide an important source of groundwater supply.
So my research performs a "follow the water" analysis of water flows
and stores to quantify the impacts on these successive elements of the
cycle. Climate Aspect Characterization
As mentioned, precipitation aspects such as more
intense rainfall events could induce impacts on water supply by channeling more water into stormwater runoff,
leaving less for surface water base flow and for groundwater storage. In fact, there are a lot of potentially important ways to
characterize
precipitation that need to be considered [Thorp
1982], [Trenberth et al. 2003], [Hennessey et al. 1997], for example:
- length of precipitation season (for regions with
distinct
seasonality)
- number of identifiable storms in the season
- length of storms [duration in hr]
- delay time between storms [hr or days]
- average intensity of storms [precipitation rate
in
mm/hr]
- maximum intensity [mm/hr]
Other
non-precipitation climate aspects like air temperature,
humidity,
wind speed, and radiation could help determine the rates of surface
evapotranspiration. Higher temperatures might also be expected to
change evapotranspiration by increasing the growing season length. The
type and condition of
vegetation is another factor in transpiration and might also be
relevant. The sea level rise expected from climate change [Meehl 2005],
[USGS 2000] might worsen the effects of saltwater intrusion and modify
submarine
groundwater discharge. Therefore, my analysis on the impacts to water cycle
elements is performed separately afrom each of these climate aspects.
Research Outline
It is not sufficient to
consider each climate
aspect and to determine just its qualitative relationship or even to
rank its relative
impact on each water cycle element or resource. To fully understand the
system and produce useful information, the research must produce
quantitative statements such as "a 20% reduction in
precipitation
produces a 70% reduction in groundwater recharge".
Here is a detailed list of the research tasks being performed:
- Select an appropriate combined surface water and groundwater model software to
handle all elements and aspects of interest. Study surface water models and assemble a list of each package's capabilities and
limitations. [DONE, choosing the GSFLOW model of USGS, click here for details]
- Document how the elements of the water path are impacted upon a simple increase or
decrease of the total annual precipitation. This
task requires a model, ideally through acquisition for some
actual physical site. Drive this model with some real
precipitation record and then again with the same record increased or
decreased by some factor. [IN PROGRESS using the USGS Sagehen Creek model]
- Vary the amount of the simple increase or
decrease of the precipitation in a linear
progression, i.e. +1%, +2%, +3%, etc. Record the partitioning of the
hydrological system impacts and establish each impact as a linear progression, quadratic, exponential, etc.
- Perform a stochastic Monte Carlo variation of important model
parameters such as conductivity and specific storativity across some
relevant range. Gather the ensemble results and compute a level of confidence
for each statement of the amount of impact on some water element.
- Repeat the previous tasks for changes from other precipitation and climate aspects.
- Repeat the analysis above with models for other existing sites to show that these
methodologies work in a variety of locations and to
demonstrate how responses vary in different settings.
- Repeat
the above tasks by driving the hydrological models with outputs from a
climate model(s), hopefully of local regional scale.
- Derive
mathematical procedures that can characterize precipitation outputs from climate models
in terms of the precipitation aspects like intensity, season length, etc.
Note
that some of task steps listed above could easily be reprioritized and
swapped to change the order to be performed. For example, I could drive
the system with climate model outputs before I investigate models for
other sites.
The results from this research are numerical impacts on
evapotranspiration, soil water content, runoff, stream flows,
groundwater resource levels, etc. for numerical changes of
precipitation, storm intensity, temperature, etc. These results are not
only useful for water resource planning, but perhaps also for future
assessments of crop growing conditions, forest and rangeland health,
flood risks, sediment transport, water quality, and various other
ecological and environmental conditions.
Finally, the other important topic not mentioned yet is climate
impact on the demands for water. This includes the increased irrigation
usage caused by higher temperatures and
evapotranspiration, agriculture double cropping becoming triple
cropping, etc. Such demand changes would be particularly complex to
model and
quantify since they often incorporate a large degree of human
choice. Human decision making is notoriously non-linear or even
unpredictable and so it is quite difficult to quantify and to model. I
still would like
to investigate such water demand impacts. But my plan is to first
develop a rigorous solution and understanding of the impacts on water
supply. Only after that is completed would I start to consider climate
impacts on water demand.
Bibliography
Thorp, John M. and Bryan C. Scott, Preliminary
calculations of
average storm duration and seasonal precipitation rates for the
northeast sector of the united states, Atmospheric Environment (1967),
Volume 16, Issue 7, 1982, Pages 1763-1774.
Allison, G.B.; Gee, G.W.; Tyler, S.W. (1994).
"Vadose-zone techniques for estimating groundwater recharge in arid and
semiarid regions". Soil Science Society of America Journal 58: 6-14.
OSTI:7113326.
Bond, W.J. (1998). Soil Physical Methods for Estimating
Recharge. Melbourne: CSIRO Publishing.
Allison, G.B.; Hughes, M.W. (1978). "The use of environmental chloride
and tritium to estimate total recharge to an unconfined aquifer".
Australian Journal of Soil Research 16: 181–195. doi:10.1071/SR9780181.
Laury Miller and Bruce C. Douglas (2004). "Mass and
volume contributions to twentieth-century global sea level rise".
Nature 428: 406–409. doi:10.1038/nature02309.
Cazenave, A.; Nerem, R. S. (2004). "Present-day sea
level change: Observations and causes". Rev. Geophys 42: RG3001.
doi:10.1029/2003RG000139.
Hennessy, K.J., J.M. Gregory, and J. F.B. Mitchell.
1997. Changes in Daily Precipitation
under Enhanced Greenhouse Conditions. Climate Dyn., 13:667–680.
Trenberth, K.E., A. Dai, R.M. Rasmussen, and D.B.
Parsons. 2003. The Changing
Character of Precipitation. Bulletin of the American Meteorological
Society, 84(9):
1205–1217.
Meehl, G.A., W.M. Washington, W.D. Collins, J.M.
Arblaster, A. Hu, L. E. Buja,
W.G. Strand, and H. Teng. 2005. How Much More Global Warming and Sea
Level
Rise? Science, 307:1769-1772.
U.S. Geological Survey. 2000. Sea Level and Climate,
USGS Fact Sheet 002-00.
Available at: http://pubs.usgs.gov/factsheet/fs2-00/.
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![[Evapotranspiration diagram]](http://en.wikipedia.org/wiki/File:Surface_water_cycle.svg)
![[Vadose & baseflows]](http://en.wikipedia.org/wiki/File:Vadose_zone.gif)