Water balance for a public park ( 7 acres ) 2

[3Hugger]

Hi,
I am attempting to produce a water balance or budget for a park. Basic Equation: In (precipitation) - Out (Evapotransiration + Evaporation from 1100m2 marsh surface + infiltration (dependent on soil and density of plantings) + irrigation for season) = Change in Storage with time.
Now if I have monthly precipitation averages (20 year span) for my park area with the various plantings and soil types, would this be enought to perform a water balance for the park?
Thank you for your time in this as it is important to me.

 

[civilperson]

No, the infiltration is a variable with the parameters effected by saturated soil, frozen soil, leaf and mulch cover, slope, etc. Runoff will occur unless unique topography exists or is created, (bowl shaped or surrounded by dike).
Evaporation is not steady state: temperature, wind speed, humidity and area of water surface all cause changes. Precipitation is an enormous variable. Average rainfall with standard deviation accounted for does not allow much accuracy in predictions.
The subgrade is another variable that is hard to quantify for water transport and storage. To model the layers of soil and the flow within is an entire field of study for hydrogeologists.
A simple model is a good tool for some uses but can not reflect reality for accurate predictions.

 

[3Hugger]

Thank you for your reply. I figure that for a rought idea of a water budget, the In - Out = change in storage would be enough. The park in question is in Canada so I am more concerned about water balance during 4 months of the year. My change in storage term relates to the amount of water that is coveyed by swales and storm sewer to an underground storage tank which serves to collect and irrigate the park with rainwater. There is an impervious layer under the site as it was previously a brownfield and as such we will not be considering any baseflow or groundwater recharge. Thank you again for any comments.

 

[blueoak]

What is the balance for?

I would add a waste and management term. How active is water management. Will it have shutoff during rain events and for so long after significant events (e.g. 5 day shutoff after 0.5" rains) How good is the spread and how much runs off. This last one is typically just a percentage of applied water and rain. Also for parks a big factor can be if you system is zoned well. Hills and trees often have poorer soils making these areas dry quicker. This can lead to "over-watering" in other locations.

(As for your monthly average)You also have to be a little careful in rainfall analysis. Large events help less than lots of little so depending on you climate this can be a problem. Where is the site located. I am alway interested in the 20% worst case year for both sizing the system and budgetting in areas where swings to drought are common.

The easiest method might be to look at the rainfall data to see how often storms occur for each month and just delete those waterings.

I used to evaluate efficiencies and help for irrigation systems and at the end of the day showing someone how to feel the soil and use a shovel to get to the roots always saved the most water.

 

[3Hugger]

Thanks for the reply. I would like to perform a monthly water balance for a public park in Toronto, Canada. The irrigation system will not come on during rain events. The park is to be built and topped with planting soil to support lawns and plants. I will choose an appropriate Runoff coefficient and thus be able to find an approximate volume of water that will be channeled to the underground storage tank via swales and sewer pipe.
I obtained my rainfall data from

http://www.climate.weatheroffice.ec.gc.ca/climate_normals/stnselect_e.html

and have choosen to use precipitation data from 1991 to represent the monthly average precipitation amounts to be expected. I will be in touch with an irrigation expert that will figure evapotranspiration and irrigation amounts.
Ok I dont know if I have gotten more confused or if I have resolved my issue. Thank your again!

 

[civilperson]

A bit niggardly in supplying information bearing on the problem. Impermeable layer above a brownfield and four months/year of concern narrow the scope of possible variables. What happens to the snow/rain that occurs in the other eight months? Any underground drainage layers built in this park to ensure non-saturated root zones? How expensive is irrigation water to justify this storage cost?

 

[3Hugger]

This effort in storage of rainwater is in an effort towards sustainability...... in real dollar terms immediate savings may not be immediately realized. I will assume water storage is full by end of May and start my water budget in June when irrigation is to start. Intially the new plantings will not require as much water but by the 5th to 6th year, the amount required should become constant.

 

[blueoak]

Typically new plantings need a little more water. You have to keep that top 3" layer wet. You get a ilttle more losses for evap (wet soil layer) and also its just harder to manage only watering that much at a time. My guess for water use for a new good system would be at an 85% efficiency. So for water use as a guide divide the ET for grass divided by 0.85. With all the other variables this will be as close as you can get until it is installed. I would oversize any storage a bit if affordable.