Hydrologic Synthesis Summer Institute Session IV The Horton Index: Hydrological Partitioning and Plant Available Water P.A. Troch, P.D. Brooks, and M. Durcik Assisted 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) 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) Session IV: Hydrologic Partitioning and Plant Available Water

P = Precipitation S = Surface/Fast Runoff W = Soil Wetting E = Plant Evaporation U = Lateral/Subsurface Runoff Q = Total Runoff H = Horton Index (Troch et al.) Hydrologists (Budyko & L’vovich) have argued for competition between W&S or E&U. (strategies?) 4 Background: The Other Horton index…

Troch et al (HP) 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. Session IV: Hydrologic Partitioning and Plant Available Water

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

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 Vegetation Session IV: Hydrologic Partitioning and Plant Available Water Huxman et al (Nature)

Breaking down the Horton Index: Where can we find information about catchment ecosystems? 8 Session IV: Hydrologic Partitioning and Plant Available Water 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? Session IV: Hydrologic Partitioning and Plant Available Water

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 Session IV: Hydrologic Partitioning and Plant Available Water

Activities (think papers) Session IV: Hydrologic Partitioning and Plant Available Water 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)

Approach Students 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 Session IV: Hydrologic Partitioning and Plant Available Water

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 Session IV: Hydrologic Partitioning and Plant Available Water

MOPEX Catchments Session IV: Hydrologic Partitioning and Plant Available Water

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 Session IV: Hydrologic Partitioning and Plant Available Water

Day 1 amIntroductory Lecture and Discussions; Student-lead Summary of section 1 Day 1 pmCalculating HI across scale; Evaluating RS Data Day 2 amDeveloping HI for Seasonally Snow-covered Catchments Day 2 pmEvaluating the Role of Soils and Topography on HI Day 3 amSummarize the Snow, Soil, and Topographical Analysis Day 3 pmRevisit Questions and Define Project Teams (2 or 3) Day 4 amGroup Data Analysis by Project Teams Day 4 pmPresentation of Initial Analysis and Discussion Day 5 amGroup Data Analysis by Project Teams Day 5 pmPresentation of Secondary Analysis and Discussion Session IV, week one, July

Session IV: Hydrologic Partitioning and Plant Available Water Session IV, week two, July 20 – 24 Address Specific Questions and Hypotheses developed during Session’s I and IV

Session IV: Hydrologic Partitioning and Plant Available Water 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

Session IV: Hydrologic Partitioning and Plant Available Water