Urban Landscape Water Management Research

Effective Rainfall Project

Since 1994, the Royal Botanic Gardens Melbourne has been proactive in developing and implementing water management strategies for efficient use of water. Developing research partnerships with others has been fundamental in continuing to strive towards best practice in urban landscape water management. The documentation of a RBG Strategic Water Plan in 2006-07 highlights the importance of this research and the further development of relationships with other organisations.

One of the critical issues facing the future survival of the Gardens is the impacts of climate change, particularly upon rainfall patterns. While the current projections probably indicate an overall reduction in annual rainfall, any changes to daily rainfall intensity and duration also have considerable affects on what precipitation actually reaches the soil for plant growth or what is ‘effective'. Landscape planning and protection in the RBG is reliant on understanding the future trends in available soil moisture. This influences living collections policy, master plans, and more specifically, day-to-day decisions regarding plant selection.

Irrigation management defines ‘effective rainfall' as the proportion of rainfall that reaches and is stored in the active root-zone. This root-zone in urban landscapes is often shallow and depths of less than 20 to 30 cm are common. Effective rainfall rates of 50-75% are currently applied in Gardens irrigation scheduling. For instance, this means that for 10mm of rainfall, only 5 to 7.5mm would be considered as useful and accounted for in the soil water balance.

In mature urban landscapes such as the Gardens (over 150 years old), the overhead tree canopy has a significant influence on the amount of precipitation reaching the soil and the selection of suitable plants to grow underneath. Over 60% of the Gardens 38.6 hectares is covered by vegetation canopy (see map below).

Tree Canopy Cover Map, Royal Botanic Gardens Melbourne

The total rainfall reaching the soil (throughfall) depends on the density of vegetation, the rainfall rate, and the actual amount over time.

Since July 2007, the School of Geography and Environmental Science, Monash University (Clayton Campus) and RBG Melbourne have been studying the influence of plant canopies on rainfall reaching the soil. Four throughfall-collecting apparatus were installed during November 2007 in the Australian Forest Walk and Herbarium Bed to measure the amount of precipitation actually penetrating tree cover, and an Honours student project was completed in May 2008.  

Throughfall Collecting Apparatus in the Australian Forest Walk

The Royal Botanic Gardens is pleased to acknowledge the strong in-kind support of Monash University through Associate Professor David Dunkerley, School of Geography and Environmental Science http://www.arts.monash.edu.au/ges/staff/ddunkerley.php in the manufacture and maintenance of the throughfall collecting devices and other ancillary equipment, and the continuing sharing of expertise in hydrology.

Research into effective rainfall is one project that is complementary and synergistic with two other urban water management projects currently in progress. Soil Moisture and Irrigation Methods in a Complex Landscape is a partnership project with the University of Melbourne and Sentek Pty Ltd to study actual plant water use, and the Sustainable Management of Hydrophobic Soils Trial project is researching how to improve the wetting characteristics of water-repellent soils in the Gardens. This project is also supported by the above partnership.  

Exploring effective rainfall in the Royal Botanic Gardens, Melbourne

How the Bureau of Meteorology records rainfall:

For analysing and keeping records of weather and climate, the Bureau of Meteorology
collects rainfall data at hundreds of locations. Their recording apparatus is always set up at an open site, with no overhanging branches or tall buildings nearby that might shelter the rain gauge from rain. The clearing around such a guage has to be quite large, because rain often falls obliquely to the ground, rather than vertically, owing to deflection by wind. The raingauges used have a sharp lip, in order to collect only the part of a drop that lies inside the perimeter of the collecting funnel. The part of such a drop that lies outside the collecting funnel drains away and is not measured. What is recorded by rain gauges set up in this way is called the open field or meteorological rainfall. 

For a sports ground covered by mown grass, nearly all of rainfall reaches the turf surface, where a significant fraction is likely to be absorbed into the soil, and be used by the grass during photosynthesis and growth. A raingauge set up on a sports ground would be well located to measure open field rainfall, though on the turf itself, some water will be held (see photo). 

beads of water held on blades of uncut grass following a shower of rain.

But if we compare this with what occurs at a location beneath a dense forest, it is evident
that much more rain would land on the leaves and branches. A raingauge located on the forest floor would not catch very much rain until all the leaves above were wet and had begun to drip water toward the ground. At the end of a storm, much of the water on the wet leaves could evaporate and return to the atmosphere (see Photos). 

beads of water held on a leaf following a shower of rain. If rain continues, beads of water can roll off the leaf, or be shaken off by wind, and so perhaps fall to the ground. In calm conditions, however, the leaves dry out following rain, and the moisture returns to the atmosphere.

beads of water held on a leaves during a shower of rain. For leaves having an appropriate shape and inclination such as these, water may escape from a ʻdrip tipʼ (this process is visible on two leave near the bottom of the photo) but often these drips simply land on leaves deeper within the foliage. If rain continues for sufficiently long, drips begin to reach the ground beneath the plants.

In other words, a raingauge beneath a forest canopy would probably catch much less than the open field rainfall. The thicker the canopy of leaves and branches, the less the raingauge would catch. 

What would be caught in the raingauge is called the effective rainfall. The rain that does get through the foliage to reach the forest floor is called ‘effective' because it is this rain that can be absorbed into the soil and support the water needs of the trees. There can actually be further losses of rain caught on understory plants or on leaf litter on the forest floor, but generally these losses are less than those caused by the forest canopy. 

How much rain gets through to the ground under trees and shrubs?

This depends upon a multitude of factors, including the season, the time of day, the humidity of the air, the wind speed, and others. The two main factors however are these:

1. The abundance of the plant foliage above the ground. Standing beneath a tree and looking up gives a measure of this. A fairly open canopy will allow a lot of sky to be seen. Under a dense canopy of leaves, it may be quite shady and hard to see the sky at all. Australian Eucalyptus trees often have an open canopy. The density of a plant canopy is quantified by measuring a parameter called the leaf area index, or LAI. The LAI expresses the total collecting area of leaves and branches that lies vertically above one square metre of the ground. For many species of eucalypt, the LAI lies in the range 0.5 - 5.0 m2/m2, but much higher values are found in coniferous forests(up to 20 m2/m2).
   
   
2. How much rain falls on the day concerned. If the foliage is dense and only a small shower (perhaps 2 mm) falls, then virtually no effective rainfall will penetrate to the ground. This of course is how it is possible to stay dry when sheltering from the rain under a tree. On the other hand, if it rains for 8 hours and 30 mm of rain falls, then it is likely that more than 80% of this would arrive at the ground. Soon after the leaves had begun to drip, a person standing beneath the tree would get just as wet as if they stood out in the open. Often the drips falling from leaves are larger than the original raindrops.

Plants vary greatly in the abundance of their foliage (and may lose their leaves in the
autumn), and rain showers vary from just a trace of rain to torrential downpours. This causes an equally large variation in the fraction of the open field rainfall that becomes effective rainfall. Through the course of an entire year, many forests receive an effective rainfall that is only 60-70% of the open field rainfall. That means that at a location where the Bureau of Meteorology recorded a yearly rainfall of say 750 mm, as little as 450 mm might reach the soil and be of use to the plants. Such information is collected in many locations around the world.

The losses of rain caused by plant foliage can be measured for individual rain showers, and tallied for seasons or an entire year. Typically, results show that for small showers, effective rainfall is only about 35-40% of the meteorological rainfall, while for larger showers that can soak the foliage and cause drips to fall to the ground, the effective rainfall can reach 75-80% of the meteorological rainfall (see Diagram 1). 

Diagram 1: Illustration of the partitioning of 25 mm of rainfall falling on the dense foliage of a tree, some understory shrubs, and mulch and leaf litter on the soil itself. Hypothetical amounts of rain that might pass through each layer are shown.

Why do we need to measure effective rainfall?

The results quoted above show that open field rainfall figures such as those from the Bureau of Meteorology can give the impression that more rain is available to support plants than is actually the case. Plants can generally only make use of the rain that reaches the ground. Knowing how much rain becomes effective and available to plants is really more important to horticulture, agriculture, and forestry than knowing the open field rainfall.

Another reason for monitoring effective rainfall is that climate is changing, and this has
implications for both rainfall and effective rainfall. If conditions become warmer, effective rainfall is likely to decline, because more water will evaporate from wet leaves when it is warm than in cooler conditions. At the same time, the plants may actually need additional water, not less, in warmer conditions. This may result in more frequent water stress for some plants. Also, the rainfall across large parts of southern Australia has been below average for more than 10 years, and as a part of this change, rain seems to be arriving in smaller showers than it once did. These small showers can be caught by the foliage quite effectively, with increased evaporative losses, and the showers may thus deliver less of the effective rainfall needed by plants. On many rainy days, there  are short breaks when rain stops and later begins again. These gaps tend to reduce the effective rainfall, because they allow the foliage some time to partially dry out. Thus, when rain resumes, the leaves must be re-wetted before they will again begin to release drips that can fall to the soil.

How is the effective rainfall being measured?

In the Royal Botanic Gardens, apparatus has been set up about 1 metre above the ground, to catch the rain that comes through the tree and shrub foliage above. Large collecting troughs, about 3 metres long, are used for this work. Long troughs are used so that they stretch across the ground, partly lying perhaps beneath a gap in the foliage, and partly beneath an area of dense leaves and branches. In this way, the long troughs are able to collect a fully representative sample of the effective rainfall. The troughs are set up in groups of four, all of which feed throughfall water into a central recording device where information is collected minute by minute on how much water is arriving at the ground (see Photo). This is later compared with the results from a raingauge located nearby at a completely open site (i.e., this gauge is recording open field rainfall). Taken
together, the 20 troughs form a rain gauge for effective rainfall that is 60 metres long.

One of the large throughfall collecting systems installed beneath large trees in the Royal Botanic Gardens, Melbourne. The central cylinder contains the measuring device and data logger. The four arms slope gently to allow water to drain into the recording device, and contain mesh filters to trap any fallen leaves. Water passes through the recording device and on into the soil, so that it is not lost to the plants. 

Apparatus of this kind can be called an ‘interceptometer' because the process in which the foliage traps rain is termed canopy interception. But the simpler term throughfall collecting trough is probably preferable. ‘Throughfall' is a helpfully clear term (an alternative to ‘effective rainfall') since it reminds us that what is being measured is indeed that part of the rain that manages to fall through the foliage and branches. Some of the throughfall actually reaches the ground through gaps in the foliage, and has not been intercepted at all. This is especially true for plants with low values of the LAI. For plants with thicker foliage (and larger LAI values), much of the throughfall is actually water that has dripped from the leaves and branches. Throughfall often does not begin until it has been raining for 10 - 20 minutes (the time taken for the leaves to become fully
wet) but it continues to reach the ground for some time after rain has ended, as the foliage drips.

What has been found out concerning effective rainfall in the Royal Botanic Gardens?

The measurement of effective rainfall is still being carried out, in order to gather
information relating to the various seasons, and from small and large rainfalls. Month by month, information on the open field rainfall and the corresponding effective rainfall is made available on the internet. Click here to go to the tables of monthly data .

The Bureau of Meteorology has more than 100 years of rainfall records for Melbourne, and these have been analysed. The data show that about three-quarters of all rain days receive totals of less than 5 mm, which would suffer considerable losses to canopy interception. The long record shows that on average, only about one day per year receives more than 50 mm. Canopy interception losses would account for only a small proportion of such large rainfalls, allowing them to deliver important amounts of effective rainfall. However, in the case of very large falls, the capacity of the soil to absorb the water sufficiently fast can become a limiting factor, and in some storms large amounts of water drain away to drains and creeks, causing flood flows. Despite this, the many hours
of rain that typically fall when very large daily totals are recorded do provide an important boost to soil water and groundwater. Nevertheless, the various pathways that rainwater can follow in showers of varying size shows how complex it can be to understand fully how and when plants derive water from small or large falls of rain, and in the various seasons of the year.

An example of the information obtained in the Royal Botanic Gardens can be illustrated by examining three days of rain received on the 12th, 13th, and 14th of December 2008. Over these days, approximately 65 mm of rain was recorded. It fell at a low intensity, averaging only 1-2 mm per hour. This is typical for rain in Melbourne. The rain began in the early afternoon of 12 December, and continued with some breaks until mid-morning on 14 December. December 13 was the wettest day, with about 35 mm of rain recorded.

Examining the data collected on those days by one of the throughfall trough systems at the Royal Botanic Gardens (in the Herbarium bed) provides information on how much of the rain fell through the tree canopy.

On December 12, only about 55% of the rain arrived as throughfall. In other words, about
45% of the rain was lost to interception and evaporation from the leaves. Similarly, on the last day of rain (December 14), when the rain ended by 9:47 am, only 29% of the 10.0 mm of rain arrived as throughfall. This represents an interception loss of about 71%, which is once again a fairly typical rate of interception loss for a few hours of rain falling at about 1 mm/h.

On December 13, the wettest day, 65% of the rain arrived as throughfall. This was therefore a day with quite a high proportion of effective rainfall. This was probably helped by the leaves already being wet from the rain of the day before, and by the more intense rain, both of which encourage dripping from the foliage. Results from many countries have confirmed that in general, days with intense rain or large amounts of rain tend to result in a larger proportion reaching the ground than days with lighter rain or a smaller amount of rain.

Overall, taking these three wet days together, about 42.5% of the rain fell through the tree
canopies to arrive at the ground as throughfall, whilst 57.5% was lost to interception and
evaporation. Expressing this a different way, the interception loss reduced the total meteorological rainfall of 66.5 mm to a smaller effective rainfall of 28.3 mm. In total, 38.2 mm of rain was lost, which is a considerable amount.

The throughfall measurements from which this information was derived are being carried out by the School of Geography and Environmental Science at the Clayton Campus of Monash University in cooperation with the Royal Botanic Gardens, Melbourne.

Effective rainfall report  

September 2009
March 2009
December 2008  

Partnership Research Project - Soil Moisture and Irrigation Methods in a Complex Landscape

Plant water use in urban landscapes is inadequately understood, particularly under the threats of water scarcity from drought.

RBG Melbourne, Sentek Pty Ltd and the University of Melbourne are jointly studying plant water use through the use of EnviroSCAN® soil moisture sensing technology.

There are five main study sites with soil moisture probes dispersed around the Gardens.

This research informs improvements to irrigation practice, and the findings are also being shared with landscape managers such as local government.

To view the Stage 1 Report click on this link - Soil Moisture and Irrigation Methods in a Complex Landscape _ Stage 1 Report

To view the conference paper, click on this link Developing Water Management Strategy for Complex Landscapes

The top bars show rainfall (blue) and irrigation (pink).

Soil moisture traces (coloured lines) are shown from sensor readings at 10,20,30,50 and 70 cm depths respectively.

Figure 1 - Triangle Bed - RBG3A, Stacked Separate Level Graph

Figure 1 shows a typical stacked separate level graph from the probe in the Triangle Bed site. The tracings have been separated for clarity of use, and show the Volumetric Water Content at 10, 20, 30, 50 and 70 cm depths in the major pane. In this way, it can be seen that a series of rainfall events on 4 November 2007 infiltrated to a depth of 70cm. Because of the rain, irrigations were temporarily ceased. Drying of the upper profile over the next few weeks then caused the plants to activate deeper root systems to maintain water status.

Irrigation and rainfall events are show in the upper graph pane.

Figure 2 -  Herbarium Bed - RBG4A, Stacked Separate Level Graph

Figure 2 shows the Volumetric Water Content trace from the Herbarium Bed site. It shows a typical example of the “stepping” that occurs due to the different amounts of water extracted by the plant and natural drainage during the day as compared to the night. It also shows that there is a clearly defined ‘Rainfall Threshold’ below which no water is detected in the top 10 cm of the soil. This gives an indication of the relative losses due to canopy and mulch interception. The top pane also shows the irrigation management interventions made in response to the rainfall events.

Figure 3 - Viburnum Bed - RBG5A, Stacked Separate Level Graph

Figure 3 shows the typical “stepping” trace of water as it is extracted by the plant roots. During the day, the plant loses water which it has gained from the soil through transpiration from its leaves. During the night, transpiration essentially gives way to respiration, and most water losses from the soil are due to drainage. As data is monitored in real time by soil moisture sensors (every 15 minutes in this case), it is possible to visualize these small changes as a “step” in water status of the soil. Also demonstrated here is the plant’s ability to remove water from greater depth as it becomes difficult to remove from the surface layers. Plants vary in their ability to do this, and these results are complicated in a mixed bed planting by variations in root type and density.

Sustainable Management of Urban Hydrophobic Soils in the Royal Botanic Gardens Melbourne

Over the last decade of drought conditions, hydrophobic (literally ‘water hating') or water-repellent landscape soils appear to be one of the emerging issues for effective irrigation management in the Gardens. The impacts of climate change such as continued warming and rainfall variability may also be increasing these water-repellent soils.  

Six trial blocks with 8 treatment plots (48 in all) have been setup within the Gardens to research suitable materials for reducing water-repellency.  The effectiveness of these treatments will be measured by soil moisture sensors. Since February 2009, irrigation and rainfall events are being recorded for each respective treatment site.  

More information can be found in the below documents:  

Throughfall Precipitation Report

Sustainable Management of Urban Hydrophobic Soils in the Royal Botanic Gardens Melbourne