Serrano Creek Soil Amendments

The alarms started last year, and now the consequences of the multi-year drought are showing up in water price increases, punitive fee schedules, and even rationing.  So how do you practice your love of gardening without guilt and penalties? Is a rock garden your only option?  Will your years of hard work be lost as your plants die off in the heat of this summer?

Outdoor water use accounts for over half of water consumed in Orange County, dropping during damp rainy days, and spiking on hot Santa Ana wind days. Your indoor plumbing has become very efficient with water saving toilettes, showerheads, and appliances.  Yet the greatest potential water savings is still available.  It requires minimal expenditure, no special skills, and pays rewards for now and the future.  A 3” layer of compost placed on the surface of your garden can reduce the total water consumption for your household by 20% or more!  At the same time your garden will look better than before! 

Here is a simplified example of how compost usage can have such a dramatic effect on the water consumed. Soils in Orange County have a large percentage of clays and silts, which results in a high degree of compaction. The condition of the soil determines what soaks in and what runs off.  Water is “delivered” to your garden from rain, or by your irrigation system. Compacted soil has little air space for water to soak in (infiltrate). This rate of infiltration can be as low as 1/3” per hour.  This means that any water applied that is more than the infiltration rate, is wasted.  To give yourself a feel of how much your sprinklers spray, put a coffee can in the garden, allow your system to run normally, and then measure the depth of the water  The typical irrigation system will spray 2.5-3” per hour.  Let’s assume that our total yard is 1,500 square feet, and we water every other day for 15 minutes. The table below shows our calculations    

By applying a layer of compost on the surface, we would be able to hold the water in place giving it time to soak in.  Even though the infiltration rate (.75”) is significantly lower than the application rate (2.75”), the compost acts as a sponge holding the water in place until it can soak in.  This means that we can reduce the time of the cycle to approximately 5 ½ minutes, and we’ll be getting the same amount of water delivered to the plant roots.

 

The second loss of irrigation water is by evaporation from the soil itself.  Think of how quickly water dries on the surface of the road on a hot Santa Ana windy day. The sun’s rays heat the water that is on the surface causing it to vaporize.  Soil is similar because as the water on the very top vaporizes, the soils (through capillary) action bring new water to the surface.  How much of the water that gets in the soil actually ends up evaporating? On those hot days as much as 70% of the day’s watering is lost to evaporation.  This is in addition to the water the plants actually consume (which is call transpiration). Using our numbers from the above example, and a conservative 50% soil evaporation rate for sunny Orange County, we would expect to lose half of the 21,038 gallons that made it into the soil.  Had the soil been covered with compost, 14,080 gallons would have remained in the soil for the plants. Reducing evaporation would allow the cycle time to be reduced an additional 50% to fewer than 3 minutes.  Quite a reduction from the original 15 minutes!

In our 1,500 square foot yard example, if compost had been used, there could have been a water savings of 66,619 gallons in a single year.  This represents a significant savings! Many water districts and cities have gone to a tiered rate structures.  By reducing the amount water used, this savings you not only save money from using less water, but the rate is itself lowered for additional savings.

Since outdoor watering typically accounts for half of the water usage, in our example, total water consumed by the household (inside and out) would have dropped by 43%.  And this is with no change to the irrigation system other than the timing of the cycle.  And with no change to your existing plants!

---

Application rate:                                                2.75” / hr

Infiltration Rate                                                .75”

Excess Application Rate                                 2”

Area water                                                        1,500 sq feet

Cycles Per Month                                            15

Time per cycle                                                 15 minutes

 

                                                   Water Chart Usage (gallons)

                                                            Per Cycle        Per Month       Per Year (8 Dry Months)

Total Water Used                                643                  9,642               77,137

Water that Infiltrated into Soil            175                  2,630               21,038           

Excess Irrigation                                 467                  7,012               56,100