Effective rainfall project
Dense overhead canopy of trees and mulch layers impact on the amount of rainfall that reaches the soil and is available to plants.
Royal Botanic Gardens Melbourne is working with The School of Geography and Environmental Science at Monash University to study effective rainfall and changing rainfall patterns. The rain that falls through the trees’ canopy to reach the ground is what is known as ‘throughfall’ This measured by devices called throughfall collecting troughs and there are four of these placed in the Gardens to assist with this research.
Tree canopy cover map, Royal Botanic Gardens Melbourne
Climate change is one of the most serious issues facing the Gardens’ landscape. Current estimates suggest annual average rainfall in Melbourne could be reduced by 15% by 2070. However, changes to the rate and length of daily rainfall can also have important affects on the amount of rain that actually reaches the soil for plant use or what is known as ‘effective rainfall’.
The effective root-zone in urban landscapes is often shallow and depths of less than 20-30cm are common. In the Gardens, estimated effective rainfall rates of 50-75% of measured amounts are currently used in water budgeting. This means that for 10mm of rainfall, only 5-7.5mm would be considered as reaching the plant roots in the soil. Under trees with dense canopy these amounts can be much less as the precipitation is ‘caught’ or intercepted by foliage and then evaporated over time without ever reaching the soil. For the amounts of water that actually reach the ground, there is then also leaf litter and or mulch layers that can intercept even more of the rain so that only a relatively small proportion of water may reach the root-zone of plants.
Mature overhead tree canopies have a significant influence on the amount of rainfall reaching the soil. Over 60% of the Gardens’ 38.6 hectares is covered by vegetation canopy. The total rainfall reaching the soil surface (‘throughfall’) depends on the density of vegetation, the rainfall rate, and the actual amount of rain over time.
The School of Geography and Environmental Science at Monash University and RBG Melbourne are studying the current rates of throughfall in the landscape and the effects of changing rainfall patterns. Four throughfall collecting troughs are based in the Australian Forest Walk and Herbarium Bed.
Throughfall collecting trough in the Australian Forest Walk
Graph of soil moisture traces at various depths. It shows that rainfall amounts greater than 7mm are needed to penetrate vegetation canopy and mulch layers to recharge soil moisture
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Last updated 12 Jan 2011
