ATS 761: Land-Atmosphere Interaction Land-Surface Water Balance & Watershed Hydrology
Hydrologic Cycle on Land Land-Surface Water Balance
Please read Bonan, Chapter 11
Change in ΔS = P – E - R Runoff Storage Precipitation Evapotranspiration
Canopy Partition of Rainwater Soil Moisture • Typically > 85% “Zones” of precip reaches
I the ground as throughfall • About 10% is intercepted by full leaf canopy (less in winter) • Intercepted water evaporates very readily!
Bonan Chaptesrs 11-12 CSU ATS Scott Denning 1 ATS 761: Land-Atmosphere Interaction Land-Surface Water Balance & Watershed Hydrology
Evapotranspiration Estimation of Evapotranspiration Thornthwaite (1948) But leaf water potential depends very sensitively avg air on soil moisture! daylength # days temp
potential evapotranspiration (mm/month)
(much more about this later!)
infiltration Darcy’s Law Movement of Soil Infiltration rate Moisture in Vertical
Moist conditions on 30 May saturated depth to hydraulic conductivity wetting front
Integrated infiltration
infiltration rate (again) Progressive drydown by 13 June Set infiltration rates equal
solve for t
Wetting front on 15 June solve for i
Bonan Chaptesrs 11-12 CSU ATS Scott Denning 2 ATS 761: Land-Atmosphere Interaction Land-Surface Water Balance & Watershed Hydrology
Estimating Runoff Why Runoff (Depends on soil type, land use, land cover, soil moisture) Estimates Matter precipitation (mm/day)
“initial abstraction”
“Bucket Model” Annual Precipitation Thornthwaite and Mather (1957) minus Evapotranspiration ~ Runoff
Bonan Chaptesrs 11-12 CSU ATS Scott Denning 3 ATS 761: Land-Atmosphere Interaction Land-Surface Water Balance & Watershed Hydrology
Watersheds “Shed Water”
A watershed Watershed is all the land from Hydrology which water drains Please read Bonan, Chapter 12 through a specified channel
ΔS = P + Gin – (E + qover + qbase)
Annual Water Budgets Experimental Manipulations Effect of deforestation on evaporative demand
• Watersheds with enough precipitation to satisfy evaporative demand shed excess water as stream discharge • How might a desert watershed compare?
Bonan Chaptesrs 11-12 CSU ATS Scott Denning 4 ATS 761: Land-Atmosphere Interaction Land-Surface Water Balance & Watershed Hydrology
Saturated Soil and Maps of Soil Water Content Overland Flow
Spatial (Exponential) Variation of Runoff From Watershed Precipitation Across a Watershed With Variable Saturation and Precipitation fractional area receiving precip infiltration frequency capacity
frequency of mean precip occurrence over whole watershed The most intense rainfall occurs over the smallest area (think convective vs stratiform) Unsaturated fraction Saturated fraction
Bonan Chaptesrs 11-12 CSU ATS Scott Denning 5 ATS 761: Land-Atmosphere Interaction Land-Surface Water Balance & Watershed Hydrology
Runoff Variable Infiltration Capacity Processes Runoff can be thought of as the sum of contributions from “buckets” of different Unsaturated fraction Saturated fraction depths and widths Most runoff comes from saturation- excess except for soils with very low hydraulic conductivity (e.g., clays)
Variable Infiltration Capacity Variable Infiltration Capacity
Shape parameter “B” controls the sizes (fractional areas) of the “buckets” on previous slide
Bonan Chaptesrs 11-12 CSU ATS Scott Denning 6 ATS 761: Land-Atmosphere Interaction Land-Surface Water Balance & Watershed Hydrology
VIC Evaluation TOPMODEL
• Like VIC, but tied directly to real topography (“hillslope” concept)
TOPMODEL River Flow • River flow (“discharge”) • Start with high- under “normal” resolution digital conditions is maintained elevation model (“DEM,” by subsurface “base elevation vs lat/lon) flow” due to water table • Derive “topographic intersecting surface index” topography λ = ln(a / tanβ) • Storm discharge due to: • Histogram (freqency – infiltration excess in dist’n) of λ across watershed catchment allows – Rain falling on saturated prediction of sat’d area, areas flows into streams and rivers infilt, & runoff
Bonan Chaptesrs 11-12 CSU ATS Scott Denning 7 ATS 761: Land-Atmosphere Interaction Land-Surface Water Balance & Watershed Hydrology
Annual Flow from Major Watersheds Watersheds of the Oceans!
Amazon > all the rest combined!
Bonan Chaptesrs 11-12 CSU ATS Scott Denning 8