IRRIGATION SCHEDULING FOR SWEET PEPPER - Management, Performance, and Applications of Micro Irrigation Systems - page 174

humidity, barometric pressure, cumulative rainfall, and solar radiation [4]. Irrigation
(IRR) was applied through a drip irrigation system. The inline-type emitters produced
a flow of 1.9 L per hour per emitter at a design pressure of 10 pounds per square inch
(psi). Emitters were spaced every 30 cm. Irrigations (IRR) were scheduled based on
the estimated evapotranspiration rate as determined with Eq. (2), where: ET pan is the
pan evaporation-derived evapotranspiration, K c is the evapotranspiration crop coef-
ficient for sweet peppers [5], which varied daily; K p is the average annual value of the
pan coefficient equal to 0.78 for Isabela, PR [6]. A cumulative water meter was used to
control the gallons of irrigation water applied. The evapotranspiration term in Eq. (1)
was estimated with Eq. (3), where: K c is the crop coefficient (dimensionless) and ET o
(mm/day) is the reference evapotranspiration obtained using the Penman-Monteith
method [5], as described in Eq. (4).

IRR = ET pan = (K c *K p *Epan) (2)

ETc = K c * ET o (3)

⎝

900

( ) ( )

0.408 ⋅ ⋅ R n − G + γ ⋅ ⋅ u 2 ⋅ e s − e a

⎠

T + 273

(4) ET o =

( )

Δγ + ⋅ 1 + 0.34 u 2 ⋅

In Eq. (4): Δ is a slope of the vapor pressure curve [kPa°C -1 ], R n is net radiation [MJ
m -2 day -1 ], G is soil heat flux density [MJ m -2 day -1 ], γ is psychrometric constant
[kPa°C -1 ], T is mean daily air temperature at 2 m height [°C], u 2 is wind speed at 2
m height [m s -1 ], e s is the saturated vapor pressure and e a is the actual vapor pressure
[kPa]. Equation (4) applies specifically to a hypothetical reference crop with an as-
sumed crop height of 0.12 m, a fixed surface resistance of 70 sec m -1 and an albedo
of 0.23.
Data required by Eq. (4) were obtained from the weather station located near the
study area. Wind speeds obtained from the 10 m high tower were adjusted to the 2 m
wind speed, required by the Penman-Monteith method, by means of an exponential
relationship. Initial values of the crop coeffi cient were obtained from the literature for
sweet pepper for the initial, mature and end crop stages (FAO Paper No. 56). Adjust-
ments of K c were made during the calibration of Eq. (1) as described later in this sec-
tion. ET o was estimated on a daily basis using a spreadsheet program. The calculation
methodology is described by Allen, et al. [5]. The values of S in Eqs. (1) and (2) were
obtained from Eq. (5).

S = θ v *Z (5)

where: θ v is the vertically averaged volumetric soil moisture content over the depth Z,
obtained by multiplying the moisture content, mass-basis (θ m ), by the soil bulk den-
sity and dividing by the density of water. The soil bulk densities were obtained from
undisturbed soil cores.
Between sampling dates when measured values of θ v were not available, daily