A new method for determining the throughfall fraction and throughfall depth in vegetation canopies

A new method for quantifying throughfall fractions and depths under vegetation canopies is introduced and evaluated. The method employs the progressive loss of weight of tablets of calcium sulphate hemihydrate (‘Plaster of Paris’) set out beneath conventional throughfall collecting funnels to form ‘throughfall integrating funnels’ (TIFs). The tablets are temporarily removed for weighing after any desired sampling period, and can continue to integrate throughfall depths for extended periods (>1 year). The throughfall fraction TFf can be found directly in ‘uncalibrated TIF mode’ by comparing the weight loss of matched open-field and sub-canopy TIFs, because sub-canopy TIFs lose less weight than open-field TIFs. The diminution in weight loss is in direct proportion to the reduction in throughfall in comparison with open-field rainfall. No other field apparatus is required, and no rainfall measurements are necessary. Alternatively, weight loss can be calibrated in g/mm using a pluviograph record, and throughfall depth TFd found using ‘calibrated TIF mode’. Estimates of throughfall made using TIFs are compared with measurements made with 16 conventional throughfall troughs, from an 8-month trial in sub-alpine snowgum woodland and closed shrub heath communities in Victoria, Australia. For the plaster tablets used, the mean open-field weight loss was 0.0096 g/mm for tablets that weighed ∼30 g initially. Average TFd estimates across eight measurement periods from the two independent method differed by <1% (91.6 mm from trough data and 91.9 mm from TIFs). Average TFf was 0.84 using trough data and 0.82 using TIFs, a difference of 2%. These very close agreements, after 871.1 mm of rain, falling in events of varying depth, duration, and rain rate, confirm that TIFs yield results consistent with those from conventional instrumental measurement of throughfall. Under a low and dense shrub heath canopy, where conventional throughfall collectors could not be installed, the TIFs indicated a throughfall fraction of 0.38, providing the first estimate for this plant community. The new method is low in cost, allowing large sampling arrays to be used if required, and is the first method able to determine throughfall fraction directly. It offers several advantages over the use of funnels connected to storage vessels, including elimination of the risk of overflow and loss of data. A particularly important advantage is the small size of TIFs, whose 100 mm height permits throughfall measurement even under low vegetation canopies, where space would not permit the installation of troughs or storage drums. TIFs can also be used to record accumulated open-field rainfall. The method of integrating flow volumes by weight loss would lend itself to the gathering of other kinds of hydrologic data, including the recording of stemflow volumes, output from small runoff plots, or indeed, wherever the recording of integrated weight loss can be substituted for the use of tipping buckets or collecting reservoirs.