While flood irrigation was being used across the globe, some ancient inventors were attempting early forms of irrigation technology considered groundbreaking today. Chinese agricultural texts from the 1st century B.C. describe clay pots being buried underground and filled with water. The unglazed and porous clay allowed the water to flow directly into the dirt around the roots of the plants. Letting the water trickle directly underground prevented runoff and protected the moisture from evaporation. Today, this method of allowing small amounts of low-pressure water (2 - 20 liters/hour) to flow directly into the root zones of crops is called drip, trickle, or micro-irrigation. This method is incredibly efficient compared to its flood and sprinkler-based relatives, as the plants utilize an estimated 90% to 95% of water. In theory, a farmer would only need half the water supply to saturate the same area through drip irrigation. Preventing excess moisture lessens the likelihood of pests, as well as unwanted runoff and drainage complications. Direct application to the root zone also means that fertilizers and nutrients can be spoon-fed to individual crops in small quantities, preventing seepage and toxins from making their way downstream. A study done by the U.C. Cooperative Extension focused on tomato farmers in the San Joaquin valley struggling with salt-rich soil. Integrating a drip irrigation system helped these farmers both with the salinity of their water (the water was picking up less salt than it would if seeping through more soil) and the resulting drainage. Compared to their traditional 25 tons per acre yield using furrow irrigation, these farmers saw yields up to 40 tons per acre. While in 1987, 0% of U.S. tomato growers used micro-irrigation, the same UCCE study found that 85% of growers do, as of 2011.
Despite this method's efficiency, only around 4% of fields around the world are watered via micro-irrigation today. Installation of a drip system is labor-intensive, and the necessary equipment is expensive and requires long-term maintenance. Surface drip irrigation comes in surface lines, like drip tape, that branch out from the main source. These range in diameter from 10 to 25 mm and release water from small holes or emitters. Narrow passages lower the pressure, meaning water drips from each emitter rather than spraying.
Less commonly, some farms utilize subsurface drip irrigation (SDI). This involves burying plastic tubes around 2cm in length between 20 and 50cm deep. Again, these emit water either through holes in the pipes or through small plastic emitters. Depending on the kind of soil, the water's pattern of permeation will change. Finer soil will move water outwards and upwards, while sandier soil will allow water to soak deeper into the earth. In places where soil and irrigation salinity are a concern, salt may accumulate in the soil surrounding the wetted zone or above the drip lines themselves. How salt moves in the ground will depend on the water's salinity, the amount of water used, where the lines are placed, and the quality of the soil itself. Both surface drip irrigation and SDI can come with pressure compensating (P.C.) or non-pressure compensating emitters. P.C. emitters release water at a steady rate regardless of water pressure. This is ideal for plots with increases or decreases in elevation where water pressure could be lost or gained. Non-pressure compensating emitters will allow water pressure to change with elevation changes and are, therefore, most often found in flat fields.
Smarter Water
In recent years, smart controller technology has made significant leaps and bounds in water management and productivity. These small computers can be integrated into an irrigation system and measure outside factors like rain and evapotranspiration (the amount of water evaporating from the soil and leaves/stems of the crops). Due to their design and common center-pivot irrigation today, most of these controllers are intended for sprinkler irrigation integrations. Some controllers are connected to outside measurements of evapotranspiration or weather conditions, while others have the means to measure weather conditions remotely onsite. Some controllers utilize advanced measurements, like soil-moisture sensors (SMSs). These underground sensors measure the soil's total moisture, allowing the controller to shut off water flow when the plant's root zones are saturated. This is great for plant health, eliminating concerns like root-rot or simple under-watering, while only using the minimum amount of water necessary. Controllers and supplemental measurements can provide valuable data that can be tracked and monitored over numerous months or years. This data can provide big-picture insights into patterns and changes in water use to react efficiently and develop plans of action over the long term.
Automation and in-depth measurements can allow farmers to make on-the-spot, minute changes to actively adjust to changing conditions. Irrigation timing is based on:
- Total evapotranspiration levels from the crop
- Monitoring of overall soil moisture
- Digital irrigation scheduling (using real-time data on-site or via communication with outside systems)
- Plant monitoring, (such as measurements of the temperatures at the canopy level of the crop)
When measuring the volume of water that you’ll be using on your fields, the following will help decide how much to use:
- The soil’s total water holding capacity
- Current measurements of the soil’s total saturation
- The overall distribution of water from your irrigation system
- Measurements of your field’s contributions to the local water table
- Pay attention to your rate of water distribution - excessive concentrations of chemicals in standing water (whether in furrows themselves or while stored in a retention pond) will have negative impacts on your soil quality
- Be aware of erosion and sediment redistribution caused by your irrigation system, as well as any areas where offsite runoff may be possible
