Evaluaton of a Center Pvot Varable Rate Irrgaton System Ruxu Su Danel K. Fsher USDA-ARS Crop Producton Systems Research Unt, Stonevlle, Msssspp Abstrat: Unformty of water dstrbuton of a varable rate center pvot rrgaton system was evaluated. Ths 4-span center-pvot system was confgured wth 10 water applcaton zones along ts 766 ft-long lateral. Two experments were conducted for the unformty tests. In one test, a constant water applcaton rate (100%) was appled, and n the other, varable applcaton rates (0%, 30%, 50%, 70%, and 100%) were assgned to each zone. To catch water appled, multple water collectors were placed n two straght lnes perpendcular to the pvot travel drecton. Three control collectors wth known amounts of water were placed at the test ste to account for water evaporaton losses durng the tests. Water caught n the collectors was measured, and the center pvot coeffcent of unformty (CU H ) was calculated. Results showed a CU H of 86.5% for the constant applcaton rate test. The effect of applcaton rate on CU H was sgnfcant, wth hgher applcaton rates provdng hgher CU H values. Keywords: Irrgaton, varable rate applcaton, center-pvot rrgaton system, unformty, precson agrculture INTRODUCTION Uncertanty n the amount and tmng of precptaton s one of the most serous rsks to crop producers n the Md-South. In recent years, producers n ths regon have become ncreasngly relant on supplemental rrgaton to ensure adequate yelds and reduce rsks of producton. Increasng groundwater wthdrawal s resultng n a declne n aqufer levels across the regon. For sustanable water use n agrculture, ncreasng water-use effcency n agrcultural producton has become a serous ssue. Compared to furrow-rrgaton methods, sprnkler systems can sgnfcantly mprove rrgaton effcency, and ther use s ncreasng n the Md-South. In the most of agrcultural felds, sol characterstcs and plant growth status consderably vary wthn a feld. Plants n one locaton may need more nputs, such as water or fertlzer, than the plants n another locaton n the feld. Treatng plants dfferently based on ther needs s requred for optmzng crop yeld and qualty. Precson agrculture technologes make t possble for farmers to adjust producton nputs ste-specfcally to address the spatal varablty n the feld. Sprnkler rrgaton systems equpped wth varable rate rrgaton (VRI) controllers are now commercally avalable. Currently two prmary control methods are used to realze VRI; speed control and duty-cycle control (LaRue and Evans, 2012). The speed control method vares travel speed of the center pvot to accomplsh the desred applcaton depth, whle the duty-cycle control changes the duty cycle of ndvdual sprnklers or a group of sprnklers. Currently there s no standard method for evaluatng a VRI system capable of makng ste-specfc water applcaton for precson agrculture practces. Lmted work has been reported on the evaluaton of VRI performance yet. The accuracy and unformty of the system are essental for the success of precson rrgaton management. The objectve of ths study was to evaluate the accuracy and unformty of a center-pvot rrgaton system equpped wth a VRI zone control package. System descrpton MATERIALS AND METHODS
The center pvot VRI system used n ths research was a Valley Standard Pvot 8000 coupled wth the Valley VRI zone control package (Valmont Irrgaton, Valley, NE). The system was nstalled at a research farm of the USDA-ARS Crop Producton Systems Research Unt at Stonevlle, Msssspp n November 2011 (Fgure 1). The system was confgured wth a total length of 766 ft, wth 4 drve unts and a flow rate of 350 gpm. Fxed-pad sprnklers (Sennnger LDN, Clermont, FL) were employed wth UP3 flat medum groove pads and 15 ps pressure regulators. The dstance from the sprnkler to ground surface was 72 n. Sprnkler spacng was 108 n, and 86 sprnklers along the length of the pvot lateral were dvded nto 10 control zones based on covered surface area. The Valley zone control package ncluded 5 VRI zone control unts, a GPS recever, and software. The control unts and the GPS recever were mounted on the top of pvot towers. Each VRI zone control unt controls the duty cycle of the sprnklers n two ndependent zones by turnng on/off electrc solenod valves to acheve desred applcaton depths n ndvdual zones. The GPS recever determnes the pvot poston for dentfcaton of the control zone n real tme. VRI prescrptons can be created usng the software provded n the package and wrelessly loaded up to the system. Experment setup The system was tested under both constant applcaton rate and varable applcaton rate condtons. New plastc cups wth a 3.5-n dameter openng and 5-n depth were used as collectors to measure the depth of water appled. Each collector was taped onto a wood stake whch was nserted nto the sol (Fgures 2 and 3). The dstance between the ground surface and the collector openng was approxmately 8 n. The collectors were unformly spaced along two straght lnes perpendcular to the drecton of travel of the pvot. The angle between the two lnes of collectors was 12 degrees. In accordance wth ASABE Standard S436.1 (ASABE Standards, 2007), no collectors were placed wthn the nner 20% of the effectve radus of the pvot, 145 ft n ths case. In the constant applcaton rate test, 78 collectors were placed wth a spacng of 8 ft n each lne. In the varable rate test, 3 more collectors were added between each control zone, for a total of 105 collectors n each lne. Detals of the control zones and desred applcaton rates are presented n Table 1. To make adjustments to the collected data to account for evaporaton from collectors, three collectors contanng known amounts of water smlar to the antcpated catch were placed at the test ste. Water remanng n the control collectors was measured at the end of the test and combned wth the recorded tme to determne evaporaton occurrng durng the tests. Test procedures The constant rate test was conducted on March 15, 2012 and the varable rate test on March 26, 2012. The pvot started at approxmately 12 degrees before reachng the 1 st test lne to allow the water pressure of the system to stablze at the desred testng condtons. The applcaton depth was set at 1 n for constant rate test. For the varable rate test, the 10 control zones were randomly assgned to 5 dfferent applcaton rates; 0, 30%, 50%, 70%, and 100%. The 100% rate corresponded to an applcaton depth of 1 n. The volume of water collected n each collector was measured usng a graduated cylnder mmedately after the pvot passed the test lne and no more water from the sprnklers reached the collector (Fgure 4). The volume of water was then converted to the depth appled based on the dmensons of the collector cups. Durng the tests, the ar temperature was around 78 F. The wnd speed was approxmately 7-8 mph S. Data analyss The center pvot coeffcent of unformty was calculated usng the formula of Heermann and Hen (ASABE Standards, 2007)
CU H = 100 1 n = 1 S V V n = 1 V S p where CU H n V S Vp s the Heermann and Hen unformty coeffcent; s the number of collectors; ndcates the th collector; s the volume of water collected n the th collector; s the dstance of the th collector from the pvot pont; s the weghted average of the volume of water caught. Vp was determned as V n = 1 p = n 1 V S S The mean of the appled depth and ts dfference from the desred depth was then computed. For the varable applcaton test, appled water depths n varous control zones were calculated followng the same procedure. Appled amounts n the area between control zones were also determned for comparson wth the appled depths n the adjacent zones. An ANOVA was performed wth SAS software (SAS Insttute Inc., Cary, NC) to compare the effect of the applcaton rate on the unformty of the pvot. Constant rate test RESULTS AND DISCUSSION The water depths measured by the collectors are plotted n Fgure 5. The average unformty coeffcent of the pvot was 86.47% wth a value of 86.45% n the 1 st test lne and 86.49% n the 2 nd test lne. There were several large fluctuatons n the depth values, caused manly by the locatons where the collectors were placed. Some collectors were located very close to a pvot tre or at the end of a test lne. The mean of the depth appled was 1.05 n, wth a standard devaton of 0.18 n. Compared wth the desred depth of 1 n, the dfference between the amount appled and the desred depth was 5%. Varable rate test The unformty test results for the varable rate test are shown n Tables 2 and 3. The ANOVA test revealed that there was a sgnfcant effect of the applcaton rate assgned to the control zone on the unformty coeffcent [F(4, 9)=115.97, p=0.0001]. Very low unformty coeffcents were observed n zones 2 and 10, whch had zero applcaton rates. The unformtes n zones 3 and 7, whch had an applcaton rate of 30%, were also notceably lower than the other zones. Ths ndcated that low applcaton rates could possbly ntroduce poor unformty. Ths result was consstent wth that reported by other researchers (Perry et al., 2003) Appled depths and desred depths are plotted n Fgure 6. Applcaton amounts followed the desred values as a general trend. The means of appled depths for each zone are reported n Table 2, and agan
show that the lower the desred depth was, the greater the dfference between the desred depth and the appled occurred. Fgure 7 shows a comparson of measured depth n the zone and the depth n the adjacent areas between two zones. A gradual depth change between two zones wth dfferent applcaton rates was consstently observed. SUMMARY Applcaton of VRI technologes has great potental for farmers to optmze crop yeld and mnmze envronmental mpact. A center pvot VRI system was evaluated wth both constant applcaton rate and varable applcaton rate. Under a constant applcaton rate, a unformty coeffcent of 86.5% was observed, and the dfference between the desred applcaton amount and actual amount appled was 5%. A varable rate applcaton test was conducted wth fve dfferent applcaton rates between 0 and 100%. The system performed well n zone control, and n general, the appled water depths followed the desred rate pattern. However, the effect of applcaton rate on unformty was sgnfcant. The unformty under hgher applcaton rates was greater than that for applcaton rates 30% or less. The varaton n applcaton rates between adjacent control zones was a gradual process nstead of an deally rapd change. Ths study was prelmnary and more comprehensve evaluatons on the VRI system performance are needed. Dsclamer The menton of trade names of commercal products n ths artcle s solely for the purpose of provdng specfc nformaton and does not mply recommendaton or endorsement by the U.S. Department of Agrculture. REFERENCE ASABE Standards. 2007. Standard Number S436.1: Test procedure for determnng the unformty of water dstrbuton of center pvot and lateral move rrgaton machnes equpped wth spray or sprnkler nozzles. ASABE, St. Joseph, MI, USA. LaRue, J. and R. Evans. 2012. Consderatons for varable rate rrgaton. Proceedngs of the 24th Annual Central Plans Irrgaton Conference, pp.111-116 Colby, Kansas, February 21-22, 2012. Perry, C., S. Pocknee, and O. Hansen. 2003. A varable rate pvot rrgaton control system. The 4th European Conference on Precson Agrculture, Berln, Germany, 15-19 June, 2003.
Table 1. Confguraton of control zones and applcaton rate assgnments Span Zone Rate No. of Drops No. No. (%) Collectors Per Zone 1&2 1 70 11 27 2 2 0 12 11 2&3 3 30 11 9 3 4 50 7 7 3&4 5 70 7 6 4 6 100 6 6 4 7 30 6 5 4 8 50 5 5 4 9 100 6 5 4&Overhang 10 0 7 4 Table 2. The unformty coeffcents n varous control zones of the center pvot system. Zone Desred Measured CU H CU H CU H No. Depth (n) Depth (n) n lne 1 (%) n lne 2 (%) average (%) 1 0.70 0.83 93.82 79.78 86.80 2 0.00 0.08-18.36-30.95-24.65 3 0.30 0.31 78.58 86.09 82.33 4 0.50 0.53 86.18 84.49 85.34 5 0.70 0.72 85.51 92.19 88.85 6 1.00 0.99 91.01 85.20 88.10 7 0.30 0.52 75.54 70.29 72.92 8 0.50 0.61 91.68 69.93 80.80 9 1.00 0.96 94.00 84.48 89.24 10 0.00 0.13-43.94-44.03-43.99 Table 3. The unformty coeffcents under varous applcaton rates. (*CU H averages wth the same character are not sgnfcantly dfferent at 0.05 level.) Rate (%) CU H n lne 1 (%) CU H n lne 2 (%) CU H average (%)* rep 1 rep 2 rep 1 rep 2 0-43.94-18.36-44.03-30.95-34.32 a 30 75.54 78.58 70.29 86.09 77.63 b 50 91.68 86.18 69.93 84.49 83.07 b 70 85.51 93.82 92.19 79.78 87.82 b 100 94.00 91.01 84.48 85.20 88.67 b
Fgure 1. Four-span Valley 8000 center-pvot varable-rate rrgaton system. Fgure 2. Water collectors lned up to catch water appled.
Fgure 3. Plastc cup to be used as the water collector. Fgure 4. Water was collected and measured usng a graduated cylnder.
Fgure 5. Water depth caught by the collectors n the constant rate test. The desred depth was 2.54 cm. Fgure 6. Desred water depth and measured water depth n the varable rate test.
Fgure 7. Appled water depth n control zones and n the overlap between control zones.