FOREST Litter Aids in Conserving Water for California Farms

Water is the limiting factor in the development and growth of California. The summer flow of streams is now appropriated; ground-water supplies have been steadily lowering in response to the heavy draft of pumping for irrigation. Imported waters such as those of the Colorado River are being sought, but the chief reliance of the State must be placed in the storage of native waters of winter precipitation for irrigation in the dry summers.

Seasonal distribution of precipitation is unequal. The year’s water supply falls during three winter months, on high and abrupt mountains.  Water drains rapidly through steep canyons across low, narrow agricultural plains, until three-fourths of it escapes to the ocean before the growing season begins. The geographical distribution is no better; three-fourths of the water falls in the north half of the State, where lies one-fourth of the arable land. Southern California, which includes one-fifth of the agricultural land, comprising the rich “citrus empire," has but 1 per cent of the State’s water supply. Three to eight square miles of watershed in this region are needed to produce the water, in addition to overhead rainfall, required for 1 square mile of agricultural or urban land.

Artificial storage must be increased. Nevertheless the principal source of supply will continue to be underground water basins of California valleys filled with deep porous outwash deposits. More than 70 per cent of the irrigation waters of southern California are pumped from these basins.

Controlled experiments conducted by the California Forest Experiment Station of the Forest Service have demonstrated that the accumulated litter of fallen leaves and twigs on the soil performs the important function of keeping open the pores and seepage channels into the mountain soils. The absorption capacity of mountain areas is controlled at the soil surface. The experiments show that forest soils burned bare of litter lose ten to thirty fold more surface run-off than soils mantled with a complete cover of litter.  The reason appears to be that the forest litter prevents the beating rain drops from picking up soil particles which, on being rearranged, would clog up the pores and seepage channels into the soil. In other words, the rain water is kept clear. This function of forest litter is emphasized in the results of comparative erosion, which is greater by one hundred to five thousand fold from bared soils than from litter-covered soils. Experiment as well as experience demonstrates that clear water sinks more rapidly into the soil than muddy water. The preservation of the forest litter, whether derived from trees, or from shrubs such as compose the chaparral forests of California, is therefore of the highest importance in the absorption of rain by mountain slopes.

More important than this, however, is the rôle which forest litter plays in the artificial spreading of flood waters over porous outwash fans to sink and store such waters in deep underground basins for subsequent pumping. Waters for sinking must be regulated and clear.  Rainfall may intermittently exceed the absorption capacity of the catchment basins and cause floods. Engineering flood control and detention and desilting works are required as protection against such floods, as well as for handling normal rainfall, if the vegetative cover of the watershed is inadequate. High flood stages can not be handled in spreading operations; on the other hand quieter but muddy flood waters are not suitable for sinking into the outwash fans. Mud clogs the soil surface and renders the fan less and less pervious and finally useless for storing purposes. Experience dictates that muddy waters as well as excessive flood stages must pass to the ocean as lost waters.

The loss of litter layers as a result of forest fires or whatever other cause not only reduces the storable water but also leaves the soils exposed to excessive erosion, products of which are carried by the resultant high stages to silt up and destroy the storage capacity of flood-control reservoirs, or to cover and damage valley orchards and farm lands with sterile outwashed sands. Keeping run-off waters clear, whether for temporary detention storage in mountain slopes, for sinking into underground basins of outwash-filled valleys, or for reservoir storage, is the important function of ground litter alike from forests and from the chaparral and brush fields of California.