Notes on Irrigation Transect Data Sets
Jim Koelliker, BAE Department, October 18, 2006

These data represent the operation of the water supplementation
(irrigation) transects at Konza Prairie Biological Station since
1991.

The data sets are continuous from May 1 through September 30 of
each year.  They contain daily amounts of rain and irrigation on
both the upper and lower portions of the two transects.  In 1991
and 1992 only one transect was established.  The second one was
added in 1993.  So, for comparison purposes, the irrigations in
1991 and 1992 can be used for both the upper and lower portions
of the transects because the entire transect was irrigated at the
same time.  The irrigation amounts are the average of six
measurements from collectors placed 2 m on either side of the
line of sprinklers.  They are placed to give an average estimate
of the area under the sprinkler pattern.  With the addition of
the second transect in 1993, we divided irrigations into to parts because of the capacity of the irrigation system was adequate to do only half at a time.  We irrigate the upper portion of both transects
as one part and the lower portion of the transects as the other
part.  Irrigations must be a day apart because it takes that much
time to refill the reservoir. In 2006, the reservoir was expanded and the pump size was increased.  Now, all parts of both transects are irrigated at the same time.  The amount applied continues to be measured at the same locations on both transects.  Application amounts are consistently close in amount and we are able to apply under more ideal wind conditions with these improvements.

Nominal irrigations are for four hours and 25-30 mm is applied to
the fully-watered portion of each the transects (2 m either side
of the sprinkler line).  We irrigate whenever the soil in the
fully-watered portion of the transect has about 50 mm of storage
available for added water in the top 1.0 m of soil.  Calculations
for irrigation scheduling were initially done by estimating water
demand using estimated water-use coefficients along with
potential evapotranspiration calculated from daily meteorological
information.  We obtained the meteorological data from the KSU
Agronomy Farm at the K-State Campus; 10 km north of the
transects.  In 1994 neutron access tubes were added in the upper
part of Transect B to a depth of 1.0 m to monitor soil water
content as well.  Since 1994 a combination of irrigation
scheduling and soil water monitoring has been used.  Also, these
data have been used to examine the water-usage coefficient for
the tallgrass prairie.  Since 2001, real-time calculations of
grass reference potential evapotranspiration have been available from the meteorological station at Konza Headquarters that is only about
300 m from the transects.  In 2002 neutron access tubes were
added in the lower part of Transect A.

Irrigations usually begin on average about June 20, but we
irrigated one time in May, May 22, 1992.  The use of water is
very low early in the season and the grass does not develop a
full coverage until late June.  Peak water demand usually occurs
between July 10 and August 15 but the highest rate of use
generally lasts only about 10-15 days.  Demand for water drops
off slowly in late August until the daily minimum temperature
drops below 10 deg C.  After the first occurrence of this event,
water usage becomes very low.  This usually occurs by September 15,
but it occurred on August 18 in 2001.  The grass stays relatively
green, but water usage is small for the remainder of the season.
The latest date we have irrigated is September 17.

Water applications for the first two years now may be about 50%
more than needed because we did not understand the water use
pattern very well then.  It now appears that IF we could
supplement water such that all that we apply could be used
effectively by the system, none wasted because of subsequent
rainfall that is ineffective, that about 200 mm per year would be
all that is needed on average.  Average annual application amount
has been about 300 mm and it has ranged from 84 to 469 mm.

Plans are to continue this study indefinitely.  Trends in greater
production of annual net primary production have developed.
Also, the transects provide another location for several other
studies to use as part of their experimental variations in their
treatments.