I awoke early Friday to the boom of thunder and the spatter of raindrops through our open bedroom window. I love rain any time of year; rain in late May generally allows me to take a break in my otherwise busy schedule. In fact, I’m sitting at my kitchen table writing rather than using a planned vacation day to do ranch work. But rain at this time of year has complicated repercussions for farmers and ranchers, including foothill sheepherders!

Our annual rangelands are not irrigated; rather, we rely on rainfall in proper amounts and at the right time to grow the grass that at least some of our sheep graze for most of the year. Even the most casual observer of grass growth knows that our annual grasses grow most rapidly in March and April; a grass geek like me notices subtle changes from week-to-week and year-to-year. This year, despite merely normal precipitation, has been a grass year like no other I’ve ever experienced. Warm air and soil temperatures, and rainfall at exactly the right time, has resulted in record forage production – more than 5,000 pounds of grass per acre at the UC Sierra Research and Extension Center just to our north.

Even in the best years, however, our annual grasses must set seed and die – no amount of late spring rain (or irrigation, for that matter) will keep annual grasses growing past the point of reproduction. Once our annual forages reach maturity, they decline in nutritional value. Rain, once the grasses have turned brown, hastens this decline in nutrition. Rain at this time of year also boosts the growth of undesirable weeds, like yellow starthistle and medusahead barley.

Our irrigated pastures are another matter. We try to keep the sheep with the greatest nutritional demand (growing lambs in early summer; ewes prior to breeding in late summer) on green grass. The Nevada Irrigation District begins delivering our irrigation water in mid-April; we’ll irrigate every day for the next 6 months to keep our pastures verdant. As you might imagine, irrigating a pasture is a bit more complicated than turning on our lawn sprinklers. We try to match supply (that is, the water we deliver through our sprinklers) with demand (the amount of water required by the plants or lost through evaporation). This demand side of the equation is called evapotranspiration, or ETo. In our pastures, ETo varies through the year, depending on air temperature, humidity, wind, and growth stage of the plants. During irrigation season, I regularly check the California Irrigation Management Information System (CIMIS), which measures reference ETo at a weather station near Auburn.

Looking at measured ETo for Auburn for the last week, we needed to apply 1.43 inches of water to meet plant demand and allow for evaporation. The quarter inch or so of rain we’ve received this morning will help meet some of this demand, but we won’t turn off our irrigation system today.

Despite this complicated story, I love a late spring storm. The rain knocks down the dust and pollen (and my allergies improve, at least for a day or two). I soak up the cooler temperatures in anticipation of the hot days just around the corner. I enjoy the sound of raindrops on the roof and the excuse to stay indoors for an extra cup of coffee!