Precipitation interception in Australian tropical rainforests Measurement of stemflow, throughfall and cloud interception D. McJannet, J. Wallace, and P. Reddell Hydrologic Processes 21:

Objectives  Measurements –Rainfall –Throughfall –Stemflow  Cloud Interception –Gauge –Canopy water balance  Results  Environmental Importance

Rainfall  Average daily rain – 12.1 mm  Tipping bucket gauges  2 m height  Wind loss correction  Horizontal precipitation inputs

Throughfall  Throughfall - Precipitation that is intercepted by canopy that falls to the ground.  2 throughfall measurements systems per site  6 m by 100 mm troughs  Collection area of m 2  Leads to a tipping bucket

Stemflow  Stemflow - Vegetation- intercepted precipitation that reaches the ground by flowing down the stems or trunks of plants  Large number of different species  Spiral collar of clear vinyl tubing  All trees linked together with a tipping bucket  12 trees, basal area 1.19 m 2  13 trees, basal area 1.73 m 2.

Cloud interception gauge  Cloud Interception - Interception of cloud droplets by the canopy  Collection surface of aluminum screen  Placed under 0.8x0.8 m plywood  Measured through tipping bucket

Canopy water balance  Developed equation from one of the rainforest sites  P = 1.14(Sf+Tf)+3.58  Large storm Ci = {1.14(Sf+Tf)+3.58} – P  Small storm Ci = {(1/Cg)*(Sf+Tf)} – P  P = Precipitation, Sf = Stemflow, Tf = Throughfall, Ci = Cloud interception, Cg = Canopy gap  Closed circles – Ci, Open circles – No Ci  If precipitation falls below the line, Ci occurred

Results – Throughfall  Replicated throughfall troughs received similar amounts of water at each site  Throughfall troughs worked well under high precipitation conditions  Throughfall decreased as precipitation event time increased

Results – Stemflow  Stemflow was measured 93% of precipitation days  Stemflow was not changed by DBH and species composition  Percentage of rough and smooth bark trees did not alter the results

Cloud interception  Gauge interception was increased with wind speed  Gauge interception = 0.41*Estimated Ci  Canopy water balance was a better measure of cloud interception than the interception gauge

Rainfall vs Cloud Interception by Month

Interception  I = P + Ci – Tf – Sf  Interception rates are high for these lower montane forest sites  ~1000 mm of precipitation evaporation annually

Environmental Importance  Throughfall and stemflow sampling worked well in the remote sites tested –Even under intense periods of rain –Monthly servicing –Under more variable canopies, more throughfall gauges may be necessary –Can give a good idea about the canopy water balance of a site

Environmental Importance  Water balance vs cloud interception gauge –Canopy water balance determined Ci better than a Ci gauge –Gauge does a good job of assessing presence of cloud interception –Small storms can show no Ci through canopy water balance  All Ci held in the canopy

Environmental Importance  Cloud interception can be a large contribution to the water budget of a tropical rainforest –Up to 70% of total throughfall and stemflow –Important source of water during the dry season

Questions?