Divining Rod May 2006

• Year: 2006
• Title: Divining Rod
• Author: New Mexico Water Resources Research Institute
• Description: May 2006
• Publications - Doc Type: Other

DIVINING ROD May 2006
Civil engineering student studies wind profile in
pecan orchard
Eric Lopez, a senior civil engineering
student at New Mexico State University,
agilely climbs a 75 -foot tall tower in the
middle of a Stahmann Farms pecan
orchard, checking his instruments along
the way. Working with Dr. Salim Bawazir,
he aims to determine if the logarithmic
estimation of the vertical wind profile of
pecan trees is as accurate or reliable as
the same estimation for short agricultural
crops.
Wind is a major factor in
evapotranspiration (ET) rates, but is not
usually directly measured. Traditionally,
the vertical wind profile of short
agricultural crops has been estimated
using a logarithmic relation. In order for
the logarithmic function to estimate the
wind profile accurately, the surface of the
crop must be relatively smooth, and the
boundary layer must be under conditions
of neutral atmospheric stability. The
surface structure of pecan trees is much
rougher than short agricultural crops, such
as alfalfa. The pecan trees at Stahmann
Farms are forty to sixty -five years old,
approximately 50 feet tall, and 1.25 to
1.64 feet in diameter. There are thirty feet
between trees and sixty feet between rows,
which allow the wind to penetrate into the
canopy.
To measure the wind profile, Lopez
attached three anemometers to a 75 -foot
tall tower. They were originally located
at 48.6 feet, 52.5 feet, and 62.3 feet in
height along the tower; however, as the
growing season progressed, he moved the
anemometers to 52.3 feet, 57.5 feet, and
68.6 feet. "Because of increased foliage,
I had to raise the anemometers to get better
results," he says. A fourth sensor, at 61.5
feet on the tower, measures more
M
by Sara Ash, WRRI
Alit
accurately and provides verification for
the other three sensors.
The anemometers measure the
horizontal wind speed through the canopy,
and the vertical placement of the
anemometers along the tower provides a
vertical wind profile. A data logger,
powered by a battery that recharges with
a solar panel, collects one sample per
second from each of the anemometers and
averages the data every thirty minutes.
The data provide a daily vertical wind
profile during the day.
Lopez used the data averaged every
thirty minutes to find daily means. On
days of the year 202, 208, 209, 210, and
216, the wind profile followed the
logarithmic profile similar to that of short
h
Eric Lopez attaches a e
thermocouple wire to
the tower located at
Stahmann Farms,
south of Las Cruces,
New Mexico.
Photos by Sara Ash
crops. Since the wind profile is
logarithmic, it allows for predictions of
wind behavior within and above the
canopy of pecan trees and for estimations
of evapotranspiration, which according to
Lopez are quite beneficial.
Using a graphical procedure, the zero
plane displacement, roughness length,
friction velocity, and the shearing stress
were determined. The average roughness
length was 0.003 meter; the average
friction velocity was 0.120 meters /second,
and the zero plane displacement was 14.64
meters. These results were based only on
data from the growing season. Further
research during the off season will be
conducted to determine wind profile
changes throughout the seasons.
(continued on page 5)