Stormwater Retention and Reuse at the Residential Plot Level—Green Roof Experiment and Water Balance Computations for Long-Term Use in Cyprus

Round 1

Reviewer 1 Report

The study entitled “Stormwater retention and re-use at the residential plot level – Green roof experiment and simulations for long-term use in Cyprus”, submitted for review, presents the results of research on the quantitative assessment of retention on green roofs, the performance of two plant species under two levels of deficit irrigation as also assessment of  stormwater runoff reduction and reuse by green roof and roof-top water harvesting systems.

The topic has been comprehensively analysed. The work is an original contribution to the knowledge in the field of stormwater management by means of sustainable measures like as green roofs. The results can be used to guide practical engineering.

Authors may consider the following points and suggestions to improve the paper as given in the following:

-       It would be worth to add to the photo of green roof test bed shown in Fig. 1 a diagram of test-beds describing their kinds (including the type of substrate, plant species and number of repetitions);

-       How did the authors calculate mean absolute error? It would be worth adding this information in chapter 2.3.1.;

-       The authors claim: "For seven of the nine events surface runoff was less than 6% of the total drainage of each event. For the other two events (17 December 2016 and 9 February 2017), total drainage was low and surface runoff was 0.01 mm (39%) and 0.11 mm (86%), respectively. The data presented in Table 2 indicate other values. Please re-read the results from the table and description in the text. They should be consistent with each other !!!

-       The authors do not discuss the quality of water used for reuse, especially for  indoor gray water use. They took into consideration the entire volume of water that runoff from the sealed and green surfaces as useful for reuse. In fact, the quality of the outflows can limited the reuse of roof runoff. It is worth mention about this in the discussion chapter;

-       Simulation analyzes were made on the basis of a relatively simple equation. For these reasons, the term "model simulation" or "modelling" is a bit "exaggerated". These terms are more associated with the use of software to modelling or simulations. Please consider the change of the term.

Author Response

We would like to thank you for your valuable suggestions and comments. Our response is provided below each comment with red fonts. We have made changes in the manuscript using Track Changes to make evident what has been modified in the text.

The study entitled “Stormwater retention and re-use at the residential plot level – Green roof experiment and simulations for long-term use in Cyprus”, submitted for review, presents the results of research on the quantitative assessment of retention on green roofs, the performance of two plant species under two levels of deficit irrigation as also assessment of  stormwater runoff reduction and reuse by green roof and roof-top water harvesting systems.

The topic has been comprehensively analysed. The work is an original contribution to the knowledge in the field of stormwater management by means of sustainable measures like as green roofs. The results can be used to guide practical engineering.

Authors may consider the following points and suggestions to improve the paper as given in the following:

-       It would be worth to add to the photo of green roof test bed shown in Fig. 1 a diagram of test-beds describing their kinds (including the type of substrate, plant species and number of repetitions);

A diagram of the test beds describing substrate type, plant species, number of repetitions and irrigation treatments has been added to Figure 1.

-       How did the authors calculate mean absolute error? It would be worth adding this information in chapter 2.3.1.;

We agree with the reviewer that this could be explained better in section 2.3.1, we have corrected this as follows: We mention in line 136-139: The parameters were fitted by minimizing the mean absolute error and bias between the observed (average of 13 sensors) and computed daily soil moisture, for the November 2016 to April 2017 period.

-       The authors claim: "For seven of the nine events surface runoff was less than 6% of the total drainage of each event. For the other two events (17 December 2016 and 9 February 2017), total drainage was low and surface runoff was 0.01 mm (39%) and 0.11 mm (86%), respectively. The data presented in Table 2 indicate other values. Please re-read the results from the table and description in the text. They should be consistent with each other !!!

The data in Table 2 present the combined drainage and surface runoff, while the text referred specifically to the surface runoff. This is now clarified in the manuscript by changing the term “total drainage” to “combined drainage and surface runoff”.

-       The authors do not discuss the quality of water used for reuse, especially for indoor gray water use. They took into consideration the entire volume of water that runoff from the sealed and green surfaces as useful for reuse. In fact, the quality of the outflows can limited the reuse of roof runoff. It is worth mention about this in the discussion chapter;

The following has been included in line 289 and responds also to Reviewer 2 comment on the underestimation of collected runoff using daily time steps: The use of a daily time step in calculations for the performance of rainwater tanks has been found to underestimate roof runoff yields compared to a 6-minute time step [31]. Moreover, the water quality of the collected runoff may reduce the volume that can be used for indoor grey water use. Thus, further research can optimize the availability of collected runoff for greywater use and irrigation.

-       Simulation analyzes were made on the basis of a relatively simple equation. For these reasons, the term "model simulation" or "modelling" is a bit "exaggerated". These terms are more associated with the use of software to modelling or simulations. Please consider the change of the term.

We agree and have changed the term model/ simulation to computations where necessary to provide the clarification.

Reviewer 2 Report

A very interesting paper with many insights into roof garden operation and performance; and reuse at the lot scale. The use of irrigation of roof gardens using harvested stormwater at the lot-scale is original however the further development of the model should look to use higher resolution timesteps (6-minutes) as these have been shown to accurately reflect "tank" water level over time (see below):

Coombes PJ¹, Barry ME. The effect of selection of time steps and average assumptions on the continuous simulation of rainwater harvesting strategies, Water Sci Technol. 2007;55(4):125-33.

I would have liked to have seen was some data/figures showing wilting point/field capacity as these would relate to plant growth and moisture holding capacity of the roof garden media.

Overall, a great paper that is worthy of publication.

Author Response

We would like to thank you for your valuable suggestions and comments. Our response is provided below each comment with red fonts. We have made changes in the manuscript using Track Changes to make evident what has been modified in the text.

A very interesting paper with many insights into roof garden operation and performance; and reuse at the lot scale. The use of irrigation of roof gardens using harvested stormwater at the lot-scale is original however the further development of the model should look to use higher resolution timesteps (6-minutes) as these have been shown to accurately reflect "tank" water level over time (see below):

Coombes PJ¹, Barry ME. The effect of selection of time steps and average assumptions on the continuous simulation of rainwater harvesting strategies, Water Sci Technol. 2007;55(4):125-33.

The following has been included in line 289 and responds also to Reviewer 1 comment on the quality of the collected runoff: The use of a daily time step in calculations for the performance of rainwater tanks has been found to underestimate roof runoff yields compared to a 6-minute time step [31]. Moreover, the water quality of the collected runoff may reduce the volume that can be used for indoor grey water use. Thus, further research can optimize the availability of collected runoff for greywater use and irrigation.

I would have liked to have seen was some data/figures showing wilting point/field capacity as these would relate to plant growth and moisture holding capacity of the roof garden media.

We agree that these are important points that could be clarified better.

We have added the following in line131-133: Because of the shallow depths of the substrate, soil evaporation cannot be separated from plant transpiration. Therefore, no wilting point was established.

And in line 201-211 we reported the observed soil moisture contents as presented in Figure 2. We have added here: Soil moisture after drainage (field capacity) during the December and January rainfall events was around 20 to 22%.

In line 248, we did report the lower soil moisture limit (4%) and the field capacity (22%), as fitted from the observations.

Overall, a great paper that is worthy of publication.