TENSIOMETER BASED AUTOMATED IRRIGATION FOR TOMATO UNDER PLASTICULTURE - Management, Performance, and Applications of Micro Irrigation Systems - page 200

16.1 INTRODUCTION
More than 65% of U.S. total vegetable acreage is irrigated [14]. Although used on
approximately 7% of the total irrigated acreage in the U.S., drip irrigation is widely
used on high-value crops [15]. Improvements in drip irrigation and increases in plas-
ticulture production have prompted significant increases (> 500%) in its use over the
previous 20-30 years, [14]. Drip irrigation, if properly managed, can achieve up to
95% application efficiencies [25].
Due to increases in yield and quality, growers often over irrigate, viewing it as
a cheap insurance policy for growing fruits and vegetables. However, just 5 h after
the initiation of drip irrigation, the wetting front under an emitter may reach 45 cm
from the soil surface, effectively below the root zone of many vegetables [9]. Water
can migrate upward into the root zone through capillary action on fi ne textured soils;
however, movement decreases as texture becomes coarser. Additionally, small scale
variability in soil textures may affect water movement [18, 40]. If water reaches a clay
subsoil, upward movement into a coarser loam topsoil can be limited. Fertilizers and
pesticides may also leach below the root zone of plants grown in coarse soils when
excessive water is applied [36]. Methods for improved scheduling and management of
irrigation may increase water use effi ciency as well as potentially reduce the leaching
of agricultural chemicals.
Irrigation scheduling has traditionally been weather or soil based; although sev-
eral plant-based scheduling methods have been proposed [10, 16]. In weather-based
scheduling, the decision to irrigate relies on the soil water balance. The water balance
technique involves determining changes in soil moisture over time based on estimat-
ing evapotranspiration (Et) adjusted with a crop coeffi cient [23]. This method takes
environmental variables into account along with crop coeffi cients that are adjusted for
growth stage and canopy coverage [12]. However, irrigating based on crop Et values
may be subject to inaccuracies due to variations in local conditions and production
practices [1, 5]. Furthermore, some growers do not have access to appropriate local
weather data and the programs necessary to properly schedule irrigation.
Often soil moisture-based methods are used to schedule irrigation. Perhaps the
simplest and most common technique is the “feel method,” where irrigation is initiated
when the soil “feels” dry [1]. More sophisticated methods involve using a tensiometer
or granular matrix type sensor [20, 24, 30, 34]. These methods require routine moni-
toring of sensor(s), with irrigation decisions made when soil moisture thresholds have
been reached. This requires the development of threshold values for various crops
and soil types. Soil water potential (Ψ s ) thresholds for vegetable crops such as tomato
( Lycopersicon esculentum ) and pepper ( Capsicum spp .) have been developed [13, 30,
32, 35]. In threshold studies, Ψ s levels are maintained at a near constant level using
automated systems [30]; or the soil is wetted for a period of time then allowed to dry
out [35]. In sandy soils, high-frequency, short-duration (pulsed) irrigation events can
reduce water use while maintaining yields of tomato when compared to a tradition-
ally scheduled high-volume, infrequent irrigation [20]. Pulsed irrigation results in a
shallower wetting front shortly after the irrigation event, increasing application ef-
fi ciencies [2, 43, 44]. Although data for pulsed irrigation is available for sandy soils,