Interception Conceptual basis Importance Definition Canopy water balance Measurement Modeling T. Giambelluca GEOG 405 University of Hawai i at Mānoa Conceptual Basis Before reaching the soil, much of the water input from precipitation, fog, and some forms of irrigation is first intercepted by vegetation Water evaporated directly from wet vegetation during an event and the water remaining on vegetation after an event does not reach the soil, but is lost to the atmosphere instead Water reaches the soil by free throughfall, dripping off wet leaves and branches, and by running down the stems (stem flow). Leaf litter adds another interception layer Importance Interception reduces amount of water reaching soil Wetting of leaves inhibits transpiration Evaporation from wet canopy usually significantly greater than dry-canopy transpiration Overall ET can be significantly higher as a result of interception In some environments, interception accounts for a large proportion of total ET 1
Definition R = P I R = P (C + L) R = (T + S L) I = P R = P (T + S L) P = gross rainfall R = net rainfall entering soil I = total interception C = canopy interception L = litter interception T = throughfall S = stemflow Canopy Water Balance Measuring Throughfall Measuring Throughfall 2
Measuring Stemflow Modeling Interception Interception Importance For many plant species, transpiration is near zero when leaves are wet Wet canopy evaporation tends to be higher than transpiration Interception process can dominate the variability in evapotranspiration, as we found at a native forest site in Hawai i Volcanoes National Park Some plants may be capable of gas exchange, including transpiration, under wet canopy conditions, perhaps giving them a competitive advantage in photosynthesis Field Sites Invaded Forest Site ohia forest invaded by Psidium cattleianum (strawberry guava) Native Forest Site Metrosideros polymorpha ( ohia) Cibotium spp. (hapu u; tree fern) 3
Energy Partitioning at Native Site Energy Partitioning at Native Site Fraction of energy for ET: Fraction of energy for ET: Decreases as Rnet increases Higher for wetcanopy periods MAX: Oct Exhibits apparent annual cycle Cycle roughly corresponds to LAI cycle Next, look at departures from the annual cycle MIN: Apr Departures of LE Fraction from Annual Cycle SOIL MOISTURE LE fraction anomaly variation: Generally similar to soil moisture variation But, LE minima precedes SM minima by about 2 weeks Photo: Trade Wind Fruits How about the effects of canopy wetting? RAINFALL LE maxima and minima match rainfall match rainfall maxima and minima Wet-canopy evaporation controls departures in LE fraction from the LAIrelated annual cycle 4
Amount Relative to Dry Leaf Conditions 60% 50% 40% 30% 20% 10% 0% 58% Leaf Wetness Effect (-) 27% Leaf Wetness Effects 48% 18% PE Transpiration PE Transpiration Native Site- Ōhi a 56% No Leaf Wetness Effect 34% 56% Invaded Site-Guava 36% Canopy Water Balance Study Master s Students Ryan Mudd and Mami Takahashi Partially Wet Wet Throughfall Measurements Stemflow Measurements Single-Layer Canopy Water Balance Model RF IE Wet-Canopy Evaporation vs Transpiration 0.25 0.20 Native CWI 0.15 0.10 52% Wet-Canopy Evap Total ET Transpiration Canopy 0.05 48% FF FF 0.00 0:00 3:00 6:00 9:00 12:00 15:00 18:00 21:00 0:00 0.25 0.20 Invaded 0.15 0.10 64% Wet-Canopy Evap Total ET Transpiration TF SF 0.05 0.00 36% 0:00 3:00 6:00 9:00 12:00 15:00 18:00 21:00 0:00 Mean Diurnal Cycle June 2007-May 2008 5
Native (mm yr -1 ) Invaded (mm yr -1 ) Diff Canopy Water Balance: Native vs. Invaded IE 500 396-21% EC ET 1249 1392 +11% Total ET 961 1102 +15% Transpiration 461 706 +53% Photo: Trade Wind Fruits Implications For Regional Water Resources: Guava invasion has significant negative impact on groundwater recharge For Dynamics of Tree Invasion: High stem density and small diameter stems favor higher transpiration; improves carbon assimilation and growth? Gas exchange uninhibited by leaf wetting 6