Agriculture Reference
In-Depth Information
11.1 INTRODUCTION
There is antidotal evidence that many farmers in Puerto Rico and the developing coun-
tries do not employ scientific methods for scheduling irrigation for their crops. In-
stead, the pump is turned on for an arbitrary amount of time without knowing whether
the amount of water applied is too much or too little. Over application of water can
lead to the waste of water, energy, chemicals and money, and also may lead to the
contamination of ground and surface waters. Under application of irrigation can lead
to reduced crop yields and a loss of revenue to the grower.
There are various approaches for scheduling irrigation. One approach is to supple-
ment rainfall with enough irrigation so that the cumulative rainfall and irrigation, over
a specifi c period of time (e.g., one day, one week, one season), matches the estimated
potential evapotranspiration, which is equivalent to the crop water requirement. Poten-
tial evapotranspiration (ET
c
) can be estimated by the product of a crop coeffi cient (K
c
)
and the reference evapotranspiration (ET
o
). Traditionally, potential evapotranspiration
is derived from pan evaporation data or meteorological data from weather stations.
Another approach involves monitoring the soil moisture and applying irrigation suffi -
cient to maintain the soil moisture content within a predetermined range. In this paper
we present an approach based on applying irrigation to the crop to meet the crop water
requirements (i.e., potential evapotranspiration), but instead of using pan evapora-
tion or meteorological data, we use a remote sensing technique. The advantage of the
method is that reference evapotranspiration can be estimated at a 1 km resolution for
the entire island each day. If the relatively simple approach presented in this chapter is
used it can potentially lead to increased effi ciency of water and energy use, and help to
reduce crop water stress and losses in crop yields.
In this chapter, a set of steps are provided fi rst to estimate the irrigation require-
ment for locations within Puerto Rico. A detailed example problem is then given to
use of the method.
11.2
METHODS FOR IRRIGATION SCHEDULING
Potential crop evapotranspiration is estimated using Eq. (1):
ET
c
= K
c
ET
o
(1)
where: ET
c
and ET
o
were previous defined and K
c
is the crop coefficient. Reference
evapotranspiration is obtained from the operational water and energy balance algo-
rithm for Puerto Rico (GOES-PRWEB). Each day the operational algorithm produces
a suite of 24 hydro-climate variables, which are available to the public on the internet.
Estimates of reference evapotranspiration are available for three widely used methods,
namely: Penman-Monteith [5], Priestly Taylor [10] and Hargreaves-Samani [5]. Of
the three methods, the Penman-Monteith method is generally regarded as superior
because it takes into account the major variables which control evapotranspiration [1],
and the method has been rigorously validated under diverse conditions throughout the
world [9]. The Penman- Monteith method is given by the Eq. (2):
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