**3. Measurement of evapotranspiration from green infrastructure**

Depending on the configuration, inflow and irrigation, climate, and the microscale hydraulic, thermal, and aerodynamic contexts, observed evapotranspiration from the

**115**

mental factors.

*Evapotranspiration from Green Infrastructure: Benefit, Measurement, and Simulation*

same type of green infrastructure can vary case by case. Based on the existing observations (excluding modeling results), ET of a bioretention unit generally varies within

perspective, ET was observed to be able to remove 0.4–70% of inflows from a bioretention unit [67, 68, 80, 88], 58–72% of inflows from a green roof unit [82, 84, 89], and

Similar to observation tasks for other landscapes, the ET measurement methods for GI can be divided into mass-balance tracking, meteorological observation, and biological diagnostic. Among them, mass-balance tracking is most often adopted due to its simplicity and cost-effectiveness. Mass balance can be tracked indirectly by interpreting the variations in moisture content or ponding water such as in permeable pavement [85], green roof [90], and bioretention cases [65] or, more often, directly monitored by the weight change via a lysimeter. These methods generally focus on a small piece of GI and by various degrees block moisture, momentum, and energy exchanges between the monitored piece and the unmoni-

Weighing lysimeter has been widely used to measure ET for major GI types, e.g. bioretentions [80, 83], green roofs [75, 78, 83, 84], and permeable pavement [86, 87]. It uses a load cell to monitor the total mass change of the container holding the GI sample. Because only the mass readings are recorded, this technique requires extra observations to distinguish the weight changes caused by ET from the changes caused by the wetting events (rainfall, irrigation) or other possible loss terms (drainage, percolation). Drainage and percolation are often difficult to measure with the matching accuracy and temporal resolution as the load cell readings. Traditional tipping bucket is designed for rainfall measurement. Its funnel collector and tipping container can be easily overwhelmed by the massive flows from the lysimeter's underdrain. So although a tipping bucket can record the occurrence and possibly the timing of the outflow events, its volumetric readings are usually unreliable. A pressure transducer can be useful for measuring still water with enough depth and open water surface but is not helpful for detecting the shallow drainage water usually collected in a container that needs to be released after each event. For each container with a different shape, the water depth sensor would need a re-calibration. Considering the difficulty of tracking drainage and percolation, the common workaround is only analyzing the lysimeter time series during the dry spells when the water balance only has ET and the change term remaining (without

Besides the state change, vapor fluxes through a part of a plant, a closed chamber, a building's footprint, and a neighborhood can be directly monitored and used to estimate ET from GI by the means of sap/leaf flux sensor [17], gas-exchange chamber [47, 78, 81, 89], eddy covariance technique [82], and airborne remote sensing [91], respectively. Both sap/leaf flux sensors and closed chambers provide a decisive way to examine the fundamental theories behind ET models. But they can only examine the flux exchange within a very limited space; the former can only measure a piece of a plant, while the latter can hold a volume up to 0.12 m3

81, 89]. The observed ET rates by these two methods are also (if not more) hardly to upscale compared to the mass balance methods due to the variations in environ-

Eddy covariance technique quantifies the surface-atmosphere flux exchanges from a certain surface area at the upwind side of the measurement sensor (flux footprint), which should not include a large fraction of unwanted land covers. This requirement poses practical challenges for using it to monitor ET from a single

[79, 80], ET of a green roof unit generally falls within the

[49, 81–84], and the evaporation of a permeable pave-

[85–87]. From the water budget

[47, 78,

*DOI: http://dx.doi.org/10.5772/intechopen.80910*

ment unit after rainfall is generally 0.5–1.5 mm day−1

2.4–30% from a permeable pavement unit [85, 86].

the range of 2–9 mm day−1

tored environment.

other inflow and loss terms).

range of 0.003–11.38 mm day−1

*Evapotranspiration from Green Infrastructure: Benefit, Measurement, and Simulation DOI: http://dx.doi.org/10.5772/intechopen.80910*

same type of green infrastructure can vary case by case. Based on the existing observations (excluding modeling results), ET of a bioretention unit generally varies within the range of 2–9 mm day−1 [79, 80], ET of a green roof unit generally falls within the range of 0.003–11.38 mm day−1 [49, 81–84], and the evaporation of a permeable pavement unit after rainfall is generally 0.5–1.5 mm day−1 [85–87]. From the water budget perspective, ET was observed to be able to remove 0.4–70% of inflows from a bioretention unit [67, 68, 80, 88], 58–72% of inflows from a green roof unit [82, 84, 89], and 2.4–30% from a permeable pavement unit [85, 86].

Similar to observation tasks for other landscapes, the ET measurement methods for GI can be divided into mass-balance tracking, meteorological observation, and biological diagnostic. Among them, mass-balance tracking is most often adopted due to its simplicity and cost-effectiveness. Mass balance can be tracked indirectly by interpreting the variations in moisture content or ponding water such as in permeable pavement [85], green roof [90], and bioretention cases [65] or, more often, directly monitored by the weight change via a lysimeter. These methods generally focus on a small piece of GI and by various degrees block moisture, momentum, and energy exchanges between the monitored piece and the unmonitored environment.

Weighing lysimeter has been widely used to measure ET for major GI types, e.g. bioretentions [80, 83], green roofs [75, 78, 83, 84], and permeable pavement [86, 87]. It uses a load cell to monitor the total mass change of the container holding the GI sample. Because only the mass readings are recorded, this technique requires extra observations to distinguish the weight changes caused by ET from the changes caused by the wetting events (rainfall, irrigation) or other possible loss terms (drainage, percolation). Drainage and percolation are often difficult to measure with the matching accuracy and temporal resolution as the load cell readings. Traditional tipping bucket is designed for rainfall measurement. Its funnel collector and tipping container can be easily overwhelmed by the massive flows from the lysimeter's underdrain. So although a tipping bucket can record the occurrence and possibly the timing of the outflow events, its volumetric readings are usually unreliable. A pressure transducer can be useful for measuring still water with enough depth and open water surface but is not helpful for detecting the shallow drainage water usually collected in a container that needs to be released after each event. For each container with a different shape, the water depth sensor would need a re-calibration. Considering the difficulty of tracking drainage and percolation, the common workaround is only analyzing the lysimeter time series during the dry spells when the water balance only has ET and the change term remaining (without other inflow and loss terms).

Besides the state change, vapor fluxes through a part of a plant, a closed chamber, a building's footprint, and a neighborhood can be directly monitored and used to estimate ET from GI by the means of sap/leaf flux sensor [17], gas-exchange chamber [47, 78, 81, 89], eddy covariance technique [82], and airborne remote sensing [91], respectively. Both sap/leaf flux sensors and closed chambers provide a decisive way to examine the fundamental theories behind ET models. But they can only examine the flux exchange within a very limited space; the former can only measure a piece of a plant, while the latter can hold a volume up to 0.12 m3 [47, 78, 81, 89]. The observed ET rates by these two methods are also (if not more) hardly to upscale compared to the mass balance methods due to the variations in environmental factors.

Eddy covariance technique quantifies the surface-atmosphere flux exchanges from a certain surface area at the upwind side of the measurement sensor (flux footprint), which should not include a large fraction of unwanted land covers. This requirement poses practical challenges for using it to monitor ET from a single

*Advanced Evapotranspiration Methods and Applications*

reestablish the urban water budget.

**2.3 Water budget reestablishment**

the near-natural water budget [24, 35, 73].

species would be preferred [78].

between human water demand and ecosystem water demand.

for managing impervious surfaces, may also drain extra stormwater from pervious surfaces and then unintendedly result in a larger baseflow than the predevelopment condition [60]. Overcompensating groundwater recharge can lead to deleterious effects on downstream waters and ecosystem like in arid regions with intermittent and ephemeral streams [24]. Moreover, excessive recharge from GI may cause groundwater mounds, which, taking a long time to dissipate [69], endanger the foundations of other infrastructures and compromise drought resilience by promoting shallow-rooted plant systems that do not extract water from deep soil [70]. Therefore, determining the appropriate ET amount for an urban watershed is complicated and requires an overview of the complete water budget. This discussion goes beyond the viewpoint of baseflow restoration and gives rise to the emerging trend of using GI to

Type and configuration of GI can not only regulate the baseflow but also affect the rest of the water budget for a single site [71, 72]. Designing a GI unit, therefore, needs to be reviewed in a broader sense. The configuration of each GI unit, though possibly having already accomplished the local-scale objectives, can be further tweaked to target the optimum goal of a greater scale such as of an urban watershed or an urban ecosystem. Then, the baseflow regulation by GI implementations eventually turns into the redesign of the water budget, such as the proposals for restoring

Targeting water budget, however, may not be so straightforward to develop due to considerations for the integrated ecosystem management for each specific climate. From the ecological perspective, aquifer recharge might be beneficial ecologically only when the recharge amount matches the predevelopment condition [60]. So, the excessive rainwater should be harvested near the rain source [24]. However, in dry environments, ET can be dominant component of the predevelopment water budget before urbanization occurred [35]. Recovering the predevelopment ET ratio will be prohibitive in such urban settings [24]. Therefore, reestablishing a new water budget somewhere between the predevelopment and postdevelopment conditions is most feasible and beneficial for human and ecosystem water demands together. Regional water budget should be determined by the weights assigned

The new equilibrium will need to integrate multiobjectives from different perspectives. For example, for the interests in urban heat island relief, GI is designed to enhance ET process, which requires the ET-focused GI with adequate storage capacity [1, 74]. For the interests in stormwater management in wet and cold regions with excessive return flows, the ET-focused GI is recommended to maximize the runoff reduction. In semi-arid environments with intermittent but intense rain events, high ET rates also guarantee the rapid update of storage capacity between storms, though irrigation supplement may be needed [75]. For regions with low recharge rate and high groundwater exploitation rate, the percolation-focused GI with highly permeable mediums might be a better option [76, 77]. In any case when increasing irrigation demand is most concerned, GI with low ET potential or drought-resistant plant

**3. Measurement of evapotranspiration from green infrastructure**

Depending on the configuration, inflow and irrigation, climate, and the microscale hydraulic, thermal, and aerodynamic contexts, observed evapotranspiration from the

**114**

GI unit, which usually only takes a small fraction of a flux footprint and is mixed with other urban land covers with distinct thermal and hydraulic properties. The eddy covariance method can be feasible for a large GI unit that covers the majority of a flux footprint, irrespective of the unsolved energy balance closure issue. A case study using eddy covariance on an 8600 m2 green roof found that an average 70% daytime flux footprint matched the green roof surface [82]. A flux tower may become more useful to measure the total change in ET for a neighborhood scale before and after implementing GI, which will provide a critical dataset that is often lacked for calibrating stormwater and urban atmospheric models.

The challenges of measuring ET from GI were partly caused by the limitations in the current sensoring technology. To help build a database useful for future research and a wider community, field experimenters should start to record a more complete background information for a GI site, such as detailed species information [78], the surrounding impervious and pervious landscapes, and a broader field of temperature, wind, and humidity conditions that can account for advection and roughness. Meanwhile, the uncertainty information including the accuracy of measurement sensors and the selective ranges of parameters is recommended to be provided [49, 92], especially when the purpose of the observation is to improve the simulation of ET from a GI.
