Root Zone

The root length or root zone changes from crop to crop. An example of this is seen in this picture. These various row crops have roots that range in depth from 2’ to nearly 5’. This is important because these crops have access to different levels of soil moisture. This will determine how much water the plant can use before irrigation is required. This can be demonstrated by comparing the potato and alfalfa in this picture. Both plants have approximately the same leaf surface so their crop coefficients are similar. However, the alfalfa root system is 2.5 times longer than the potato root system. Therefore alfalfa will have access to 2.5 times as much water as compared to the potato grown in similar soils. The result is that the potato will need to be irrigated approximately twice as often as the alfalfa.

Perrenial crops are the main crops grown in the District.  These include stone fruits (cherries, plums, nectarines, and peaches), pome fruits (apple and pears), and wine grapes.  The IMS program uses a 3'-root zone for the tree crops and a 4'-root zone for the wine grapes.  The program has historically found that these values are the best for predicting water needs when scheduling irrigation event.  These values are modified at specific crop sites based on the soil conditions found in certain areas of the District.

The root zone can also be broken up into 4 regions of water consumption based on a percentage of the root depth. The majority of the water utilized by the plant is extracted in the top 50% of the root zone with a small amount extracted by the bottom half. This is important when cover crops are used. The cover crops used in the District have root zones that extend to 18” which is one half to one third of the root zone for most of the crops grown here. Therefore, the water utilization for the upper root zone will increase based on the amount of cover crop.