hydrologic synthesis summer institute session iv the horton index: hydrological partitioning and...

Hydrologic Synthesis Summer Institute Session IV The Horton Index: Hydrological Partitioning and

Plant Available Water

P.A. Troch, P.D. Brooks, and M. DurcikAssisted by Ben Ruddell, Siva Sivapalan, and others

Objective: To address fundamental questions linking Hydrology and Ecology in a data-rich workshop setting

Hydrology

•Where does water go when it rains?

•What controls that partitioning?

Ecosystem Ecology

•How do we quantify plant available water at seasonal/annual time scales?

•How does vegetation respond to changes in seasonal/annual precipitation?

Session IV: Hydrologic Partitioning and Plant Available Water

Can we improve hydrological, ecological, and biogeochemical predictability by introducing a reproducible measure of hydrologic partitioning into existing theory and observations?

Background: The Other Horton index…

Horton, 1933 (AGU)

H constantV

W

V : Growing-season vaporization (E+T)W : Growing-season wetting (P-S)

“The natural vegetation of a region tends to develop to such an extent that it can utilize the largest possible proportion of the available soil moisture supplied by infiltration” (Horton, 1933, p.455)



P = PrecipitationS = Surface/Fast RunoffW = Soil WettingE = Plant EvaporationU = Lateral/Subsurface RunoffQ = Total RunoffH = Horton Index (Troch et al.)

Hydrologists (Budyko & L’vovich) have argued for competition between W&S or E&U. (strategies?)

P S W

W E U

Q S U

H P Q

P S

E

W

4



Troch et al. 2009 (HP)

H constantV

W

V : Growing-season vaporization (E+T)W : Growing-season wetting (P-S)

We have indeed found that inter-annual variability in hydrologic partitioning across a range of ecosystem types and spatial scales seems to be consistent with the hypothesis that vegetation becomes more efficient in its water-use as water variability decreases.

Humidity Index

0 1 2 3 4 5 6

Ho

rto

n I

nd

ex

0.0

0.2

0.4

0.6

0.8

1.0

Humidity Index

0 1 2 3 4 5

ET

0

200

400

600

800

1000

1200

1400



How does ecological theory view the interactions between precipitation, vegetation and water use?

VPQ

EcolEnergyfV

),(

WP

6

Background: Water and Vegetation

Plant available water is equal to or a constant fraction of precipitation

V is a function of available energy and ecosystem properties (that evolve – adapt, acclimate, assemble)

Runoff is the remainder

V P

PE PE

Ecological research has focused on the control that precipitation has on productivity

But

There is significant controversy over whether this control is observable at ecosystem to regional scales (other limiting factors cloud the effects of water on plant growth)

Background: Water and VegetationSession IV: Hydrologic Partitioning and Plant Available Water

Precipitation (mm)0 500 1000 1500 2000 2500 3000 3500

ND

VI

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Huxman et al. 2004 (Nature)

Breaking down the Horton Index: Where can we find information about catchment ecosystems?

H P Q

P S

E

W

8


This activity will bring together new understanding in both hydrological and ecosystem processes, with advances in observation to address two overarching questions:

How does spatial variability in physical and ecological characteristics of catchment-ecosystems control the partitioning of precipitation?

And,

How does temporal variability in climate/ precipitation influence vegetation structure and biogeochemical function in these catchment ecosystems?

How does spatial variability in physical and ecological characteristics of catchment-ecosystems control the

partitioning of precipitation?

Example questions:1) Under what temporal and spatial scales does the

assumption of no change in storage hold? Can we expand the number of catchments (sample size) if we vary time scales?

2) Does a simple topographic index predict how P is partitioning into W and S?

3) How do soil properties influence both W and H?4) How does seasonality in precipitation influence W

and H?


How does variability in climate/ precipitation interact with vegetation structure to constrain biogeochemical

function in these catchment ecosystems?

Example Questions:1. Is forest vegetation less sensitive to drying than

herbaceous vegetation?2. Are there phenological signals associated with an

increase in the Horton Index?3. Are there temporal lags related to previous wet or

dry years apparent in Remote Sensing Indices?4. Do estimates of regional carbon uptake using the

Horton Index vary from standard MODIS products?10


Activities (think papers)


•Expand HI to include snow dominated catchments and others where storage is likely to be important to vegetation, discharge, or both. Identify the physical causes of variability in Horton Index or P-S or P-Q (soils, topography, etc.)

•Evaluate the ability of the Horton Index to improve estimates of vegetation productivity including modifying existing algorithms in Biome BGC used to calculate net carbon uptake; this includes comparisons with LTER and other flux sites

•Evaluate the ability to capture NDVI variability over a wider range of catchment types – all 431 MOPEX sites

•Evaluate the ability of the Horton Index to improve predictions of nutrient export at catchment scales. H: N export or retention is related to dryness; H: N export or retention is related to Q (amount); H: N export or retention is related to S (flashiness)

ApproachStudents will work in small teams to tackle subsets of the issues/ questions/ analysis; teams will come together several times each day to present progress, discuss implications, and refine direction

7/20-7/21 – focus on understanding physical controls on H

7/22-7/24 – focus on evaluating how ecological patterns and processes are related to H

7/27-7/29 – evaluating models from session I and addressing hypotheses from session IV


A Cautionary Note on Remote Sensing Products

There is a wide range of remote sensing products available (NDVI, LAI, EVI, ET, PSN, GPP).

Our primary focus will be on direct observations of reflectance (e.g. NDVI), and much less so on more advanced/ derived products

These products rely on model calculations that make assumptions about water availability and precipitation that may not be consistent with the Horton Index

Remote sensing provides catchment wide estimates of vegetation structure and function in response to available water


MOPEX Catchments


Research Catchments and FluxNet

Andrews

Baltimore

Coweeta

Hubbard Brook

Konza

Shortgrass Steppe

Niwot Ridge

Sevilleta

Phoenix

LTER sites

FluxNet sites

LTER and FluxNet sites

Walnut Gulch

Blodgett Forest

Duke Forest

Bartlet Ex. Forest

Cedar Bridge

Valles Caldera Walker Branch

Jasper Ridge

S. CA climate grad.

Santa Rita Mesquite

Freedman Ranch



Day 1 am Introductory Lecture and Discussions; Student-lead Summary of section 1

Day 1 pm Calculating HI across scale; Evaluating RS Data

Day 2 am Developing HI for Seasonally Snow-covered Catchments

Day 2 pm Evaluating the Role of Soils and Topography on HI

Day 3 am Summarize the Snow, Soil, and Topographical Analysis

Day 3 pm Revisit Questions and Define Project Teams (2 or 3)

Day 4 am Group Data Analysis by Project Teams

Day 4 pm Presentation of Initial Analysis and Discussion

Day 5 am Group Data Analysis by Project Teams

Day 5 pm Presentation of Secondary Analysis and Discussion

Session IV, week one, July 20 - 24


Session IV, week two, July 20 – 24

Address Specific Questions and Hypotheses developed during Session’s I and IV


What to Bring

•Curiosity and creativity

•Laptop

•Database and Statistical software (e.g. JMP)

•Graphing software (e.g. Sigmaplot)

•GIS software (if familiar)

•Matlab

•Coffee cup

hydrologic synthesis summer institute session iv the horton index: hydrological partitioning and...

Documents

plant available waterp

plant available waterhow

plant available waterhuxman

plant available watercan

partitioning of precipitation

hydrological partitioning

plant evaporationu

plant growthbackground