Agriculture Reference
In-Depth Information
capacity of the soil. In this method, if the water added to the profi le by rainfall or ir-
rigation exceeds the soil moisture holding capacity (or fi eld capacity), then the excess
water was assumed to be equal to PERC and the moisture content was set equal to the
fi eld capacity on that day. This approach has previously been used for irrigation sched-
uling [7], waste landfi ll leachate estimation [8] and estimation of aquifer recharge rates
[9, 10]. In this study, the effective fi eld capacity of the soil was determined in-situ by
saturating the soil and obtaining the soil moisture content within 48 h.
Calibration of the water balance equation was accomplished by adjusting the ratio
of runoff to rainfall (RO/R) within reasonable limits, until the measured and estimated
soil moisture content were in reasonable agreement. [1 - (RO/R)] represents the frac-
tion of rainfall that infi ltrates into the soil bed. This contribution of water can occur in
several ways for the plastic covered bed-type system used in this study. Rainfall may
enter directly through the holes in the plastic made for the plants. Rainfall that runs off
of the plastic into the furrow or that falls directly into the furrow may also be absorbed
into the beds. Under fl ood conditions, which occurred on several occasions during the
two crop seasons, water could have entered the beds under a positive water pressure.
For nonfl ooding rainfall events, soil water may move from the furrows into the beds
by means of unsaturated fl ow, which is controlled by the pore water pressure gradient
between the furrow and the bed.
14.2.3 NITROGEN LEACHING
Nitrogen leaching (nitrate and ammonium) was estimated by multiplying the daily
value of PERC by the concentration of nitrogen within the 60 to 80 cm depth of soil.
This vertical interval was considered to be below the root zone, since plant roots were
not observed within this interval any time throughout the two seasons. The following
Eq. (6) was used to estimate nitrate and ammonium leaching, respectively:
LNO
3
= [0.01*(ρ
b
)*(NO
3
)*(PERC)]/ [θ
vol
]
(6a)
LNH
4
= [0.01*(ρ
b
)*(NO
4
)*(PERC)]/ [θ
vol
]
(6b)
where: LNO
3
and LNH
4
are the kg of nitrate and ammonium leached below the root
zone per hectare, NO
3
and NH
4
are the nitrate and ammonium soil concentration in
mg/kg in the 60 to 80 cm depth interval, PERC is the percolation rate in mm, ρ
b
is
the bulk density (gm/cm
3
), and θ
vol
is a volumetric moisture content (cm
3
/cm
3
) in the
60 to 80 cm depth interval. Equations (6a) and (6b) were used on a daily basis. Each
measured value of soil concentration used in Eqs. (6a) and (6b) were based on the
average of four replications. Values of NO
3
and NH
4
between sampling dates were
linearly interpolated.
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