Discussion # 5 (22 July 2009)

Article Reference
Q. Xiao, E G McPherson, J R Simpson and S L Ustin, Hydrologic processes at the urban residential scale, October 2006

Introduction
The paper focuses on building up a physical numerical model to understand hydrologic processes in the urban scale and its interaction with different BMP’s. The model was applied to a single family residential lot in Los Angeles, CA. Model was calibrated and validated by the data collected for over 2 yrs. They have modeled rain gutters, driveway interceptors and lawn retention basin. It was observed that there was about 65% reduction in storm water by using driveway interceptor. But using the rainwater gutters the annual irrigation demand was provided by 9%. They modeled trying to replace the existing loam soil with clay soil and it was observed that annual runoff discharge to the street was increased by 63%. In this study they have considered the case study of Los Angeles which has the polluted runoff being flown into local watershed and thereby into the coastal recreation areas. LA experience sever summer deficit of water demand where they import almost 85% of the water to meet their demands. Author focuses on how the policy makes are considering the implementation of the decentralized approach in the urban watershed i.e creating a mini reservoir system which retains runoff and reducing the irrigation demand but they lack the quantitative data on the effectiveness of the usage of different BMP’s. Most of the urban hydrologic models focus on the water balance and not on the effects on the BMP’s on the hydrologic processes which are dependent on the land use.

Objective of the study:
Develop, calibrate and validate the physical model at the residential scale for simulating the runoff for both long term events and storm term storm events. The study is carried out to see the effectiveness of BMP and simulating the effects of retrofitting the control site with the BMP’s.

Methodology:
• Site: residential housing unit
• BMP’s considered: cistern, swale, rain gutter, lawn retention basin, driveway interceptor and drywell
• Designed for 50 yr storm event
• Runoff from rooftops flowed into cistern and lawn retention basin
• Runoff from driveway flowed into the interceptor and further into the drywell

The data was collected from Jan 2001 to Jan 2003 and the model was calibrated and modeled based on the measurements of land covers, hydrologic processes and micro climate data. The runoff from treatment Site Street was collected in the measurement system and then through the water outlet drainages where as the runoff from the control site was drained directly into the drive way and thereby into the street. Each land cover was considered as a individual modeling unit. The model was studied for both long term simulation and short term events. For the long term simulation the focus of study was taking into consideration the annual irrigation use and runoff to the street and for short term simulation the focus was on the storm runoff to the street and the storage of the BMP.
• Precipitation data taken as the input of the rainfall data
• Two strategies of irrigation (first using the annual ET and second one was based on the soil and water deficit) were used in the model to determine the timing and the amount of water to be applied.
• Interception losses are also taking into consideration in the model
• Surface flow is calculated using the manning’s equation
• Infiltration is estimated using Green Ampt’s equation
• Percolation
• Evapo-transpiration
• Lateral subsurface flow
• Cistern storage is based on the total amount of water entering the cistern and the cistern restricted area

Numerical simulation focused on the landscape irrigation, storm runoff, ET and percolationof runoff into the deep layers, sensitivity analysis of the landscape retrofit was simulated using the date of 2001 and the 50 yr storm event (1988). The purpose of the sensitivity analysis was to distinguish which BMP was more effective with respect to runoff reduction and conservation of landscape irrigation water use. For the control site there were not BMP retrofitted which was considered as the base condition and the treatment site was varied with 6 different BMP’s. Other type of sensitivity analysis was carried by replacing with the clay soil instead of sandy soil.

Results:
Both treatment site and control site had about 50% impervious cover. Field measurement revealed that both cistern and retention basin were more effective in conserving municipal water supplies and reducing the total storm runoff. Surface runoff was effectively reduced by the drive way interceptors and lawn retention basin.

Discussion:
It was observed that all the BMP’s installed in the treatment site were effective in reducing the storm runoff and reducing the irrigation demand from the municipal water demand. Using this strategies enabled the improvement of urban eco system. BMP has reduced the maximum runoff flow to the streets and the maximum annual evaporation and increased the deep percolation. Increased percolation increases the downstream base flow and groundwater recharge. For runoff reduction driveway interceptor and drywell combination was the best but this BMP carries the risk of carrying pollutants from the surface runoff into the ground water. Cistern provided additional irrigation demand supply but for more effective use of cistern it should be appropriately designed for the catchment areas. Few of the sites will not follow the sub surface balance which focuses on the importance of more detailed model with such kind of topographic situations.

Conclusions
• Most effective BMP is driveway interceptor
• Cistern provided 9% of the annual irrigation demand
• Annual irrigation demand was reduced by 53% by adjusting the irrigation efficiency and application rates based on the plant water demand
• Simulation studies concluded that infiltration and surface runoff is the function of the soil properties and its effective depth
• Selection of the BMP is based on testing the soil properties