Precipitation interception in Precipitation interception in Australian tropical rainforestsAustralian tropical rainforests

Measurement of stemflow, throughfall and cloud Measurement of stemflow, throughfall and cloud interceptioninterception

D. McJannet, J. Wallace, and P. ReddellD. McJannet, J. Wallace, and P. ReddellHydrologic Processes 21: 1692-1702Hydrologic Processes 21: 1692-1702

ObjectivesObjectives

MeasurementsMeasurements– RainfallRainfall– ThroughfallThroughfall– StemflowStemflow

Cloud InterceptionCloud Interception– GaugeGauge– Canopy water Canopy water

balancebalance ResultsResults Environmental Environmental

ImportanceImportance

RainfallRainfall

Average daily rain – 12.1 mmAverage daily rain – 12.1 mm Tipping bucket gaugesTipping bucket gauges 2 m height2 m height Wind loss correctionWind loss correction Horizontal precipitation inputsHorizontal precipitation inputs

ThroughfallThroughfall

Throughfall - Precipitation Throughfall - Precipitation that is intercepted by that is intercepted by canopy that falls to the canopy that falls to the ground.ground.

2 throughfall 2 throughfall measurements systems measurements systems per siteper site

6 m by 100 mm troughs 6 m by 100 mm troughs Collection area of 2.4-3.6 Collection area of 2.4-3.6

mm22

Leads to a tipping bucketLeads to a tipping bucket

StemflowStemflow

Stemflow - Vegetation-Stemflow - Vegetation-intercepted precipitation that intercepted precipitation that reaches the ground by flowing reaches the ground by flowing down the stems or trunks of down the stems or trunks of plantsplants

Large number of different Large number of different speciesspecies

Spiral collar of clear vinyl Spiral collar of clear vinyl tubingtubing

All trees linked together with a All trees linked together with a tipping buckettipping bucket

12 trees, basal area 1.19 m12 trees, basal area 1.19 m22

13 trees, basal area 1.73 m13 trees, basal area 1.73 m2.2.

Cloud interception gaugeCloud interception gauge

Cloud Interception - Cloud Interception - Interception of cloud Interception of cloud droplets by the canopydroplets by the canopy

Collection surface of Collection surface of aluminum screenaluminum screen

Placed under Placed under 0.8x0.8 m plywood0.8x0.8 m plywood

Measured through Measured through tipping buckettipping bucket

Canopy water balance Canopy water balance Developed equation from Developed equation from

one of the rainforest sitesone of the rainforest sites P = 1.14(Sf+Tf)+3.58P = 1.14(Sf+Tf)+3.58 Large storm Ci = Large storm Ci =

{1.14(Sf+Tf)+3.58} – P{1.14(Sf+Tf)+3.58} – P Small storm Ci = Small storm Ci =

{(1/Cg)*(Sf+Tf)} – P{(1/Cg)*(Sf+Tf)} – P P = Precipitation, P = Precipitation,

Sf = Stemflow, Sf = Stemflow, Tf = Throughfall, Tf = Throughfall, Ci = Cloud interception, Ci = Cloud interception, Cg = Canopy gapCg = Canopy gap

Closed circles – Ci, Closed circles – Ci, Open circles – No Ci Open circles – No Ci

If precipitation falls below If precipitation falls below the line, Ci occurredthe line, Ci occurred

If precipitation falls below If precipitation falls below the line, Ci occurredthe line, Ci occurred

Results – Throughfall Results – Throughfall

Replicated throughfall troughs received similar amounts of Replicated throughfall troughs received similar amounts of water at each sitewater at each site

Throughfall troughs worked well under high precipitation Throughfall troughs worked well under high precipitation conditionsconditions

Throughfall decreased as precipitation event time Throughfall decreased as precipitation event time increasedincreased

Results – StemflowResults – Stemflow

Stemflow was Stemflow was measured 93% of measured 93% of precipitation daysprecipitation days

Stemflow was not Stemflow was not changed by DBH and changed by DBH and species compositionspecies composition

Percentage of rough Percentage of rough and smooth bark trees and smooth bark trees did not alter the resultsdid not alter the results

Cloud interceptionCloud interception

Gauge interception Gauge interception was increased with was increased with wind speedwind speed

Gauge interception = Gauge interception = 0.41*Estimated Ci0.41*Estimated Ci

Canopy water Canopy water balance was a better balance was a better measure of cloud measure of cloud interception than the interception than the interception gaugeinterception gauge

Rainfall vs Cloud Interception by MonthRainfall vs Cloud Interception by Month

InterceptionInterception

I = P + Ci – Tf – SfI = P + Ci – Tf – Sf Interception rates are Interception rates are

high for these lower high for these lower montane forest sitesmontane forest sites

~1000 mm of ~1000 mm of precipitation precipitation evaporation annuallyevaporation annually

Environmental ImportanceEnvironmental Importance

Throughfall and stemflow sampling worked Throughfall and stemflow sampling worked well in the remote sites testedwell in the remote sites tested– Even under intense periods of rainEven under intense periods of rain

– Monthly servicingMonthly servicing

– Under more variable canopies, more Under more variable canopies, more throughfall gauges may be necessarythroughfall gauges may be necessary

– Can give a good idea about the canopy water Can give a good idea about the canopy water balance of a site balance of a site

Environmental ImportanceEnvironmental Importance

Water balance vs cloud interception gaugeWater balance vs cloud interception gauge– Canopy water balance determined Ci better Canopy water balance determined Ci better

than a Ci gaugethan a Ci gauge

– Gauge does a good job of assessing Gauge does a good job of assessing presence of cloud interceptionpresence of cloud interception

– Small storms can show no Ci through canopy Small storms can show no Ci through canopy water balancewater balance All Ci held in the canopyAll Ci held in the canopy

Environmental ImportanceEnvironmental Importance

Cloud interception can be a large Cloud interception can be a large contribution to the water budget of a contribution to the water budget of a tropical rainforesttropical rainforest– Up to 70% of total throughfall and stemflowUp to 70% of total throughfall and stemflow

– Important source of water during the dry Important source of water during the dry seasonseason

Questions?Questions?