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Experiments conducted at UCSC have been used to estimate how much lettuce could be grown with a 500 gallon (1892 liters) rain tank over the Santa Cruz dry season. With a tank this size, we can fully and efficiently irrigate two 4'x8' garden beds over the dry season from between April and September and grow roughly 672 mature lettuce heads (more than 100 lettuces per month)!
Proper irrigation is hard!
It is very easy to over water or underwater. We installed humidity measuring devices so we could know exactly how much water was used. We learned from this that even a seasoned gardener can easily over water! This means that precious water is being lost and the water in the tank won't last as long. We are working on guidelines for home scale irrigation and will update this page as we continue this work.
How to irrigate with a raintank
In the USA, rainwater in tanks is classified as non potable. This means that the water should not go directly on vegetables. Instead, regulations for water from a raintank require drip irrigation so that the water does not touch the produce. However, unless your tank is fairly high up on a hill, you are unlikely to have enough pressure from gravity alone to irrigate with drip irrigation. In our case, we also wanted to measure the water flow using a flow meter. However, even though we purchased a fairly high end $250 flow meter, we found that it did not work at low water pressure levels. In the end, we found it easier to water with a watering can. We marked a watering can to indicate how many liters were used. We found this method the most straightforward to track water usage. When watering, we did not use the sprinkler head and were careful to only add water between vegetables, not on the vegetables themselves.
How Much To Irrigate
The ideal way we found to calculate how much to irrigate our lettuce was through a weather station service called the California Irrigation Management Information System from the California Department of Water Resources, or CIMIS for short. Through collected data over our 5 month trial period in the late spring and summer of 2024 we have found the strategy which we will illustrate below, to be the most efficient method in using the right amount of water for a successful yield while conserving water use.
CIMIS is an open source, government funded service and website which draws information from weather stations around California and provides evapotranspiration (ETO) and other meteorologic data in a timely and accesible way. We relied on the Delaveaga weather station (station ID 104) for our ETO data, opperating on a 3 times a week watering schedule, watering Monday, Wednesday, and Friday. We used an ETO replacement irrigation strategy meaning we summed up the total ETO (in millimeters) measured from Delaveaga each day since the last day we irrigated and then irrigate our lettuce with that much water to replace the water lost from evaporation and transpiration.
To turn ETO millimeters into liters we first found the area of our bed to be 29760 cm^2 simply by measuring.
We then multiplied this by 1mm (or 0.1 cm) to get a conversion factor,
29760*0.1cm = 2976 cm^3.
One Liter is equal to 1000 cm^3 so we divided by 1000 to get the 2.76 liters per millimeter of ETO lost.
We adjusted these numbers depending on the growth stage of our lettuces. Larger lettuces will have more evapo-transpiration. To correct for this we multiplied the ETO by a growth scale factor based on the growth period of our lettuce variety (Newham)
After totaling all of the ETO lost (in mm) since the last day we irrigated, multiplying it by 2.76 liters per millimeter of ETO lost (which is specific to our garden bed size but can easily be recalculated), and multiplying by the growth factor scale we were able to calculate how many liters to water our lettuce!
How Much Water We Used and How Much Lettuce We Grew
Each garden bed was planted with 112 lettuce transplants which were started in a greenhouse 3-4 weeks prior to planting. Utilizing the CIMIS irrigation ETO replacement strategy demonstrated above would have allowed us to grow 672 mature lettuce heads using 2 4'x8' garden beds over three, 8 week plant out sessions between April and September. Excluding the greenhouse irrigation used during germination we used a total of 988 Liters per bed or 261 gallons per bed for a grand total of 522 gallons of water for 672 lettuce heads in the dry season. Given that our rain tank holds 500 gallons of water this means we would have theoretically able to irrigate almost fully from free collected rainwater that was captured from the wet seasons, and used during the dry summer months.
Pests
Our lettuces were raided by birds and by rats which decimated many of our plantings. They especially like to go after young seedlings. We designed several covers to keep pests out. The first was made from a plastic mesh and was too flimsy and rats chewed through it. The second was made from standard chicken wire but birds and rats could still get through the holes. We are now working on a third design that will use galvanized steel hard cloth with a smaller mesh size. We'll update here when we have the new design!
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