WARM-SEASON TURFGRASS N RATES & IRRIGATION BMP Project 2: Effect of N Rate on Nitrate-N Leaching in Established Grasses DEP WM 869 Final Project 2 Report to FDEP Oct 2009 Dr. Laurie E. Trenholm University of Florida, Institute of Food and Agricultural Sciences P.O. Box 110675 Gainesville, Florida 32611-0675 (352) 392-1831 x374, letr@ufl.edu Dr. John L. Cisar University of Florida, Institute of Food and Agricultural Sciences Ft. Lauderdale Research and Education Center (FLREC) 3205 College Ave. Ft. Lauderdale, FL 33314 (954) 577-6336, jlci@ufl.edu Dr. J. Bryan Unruh University of Florida, Institute of Food and Agricultural Sciences West Florida Research and Education Center (WREC) Jay Research Farm 4253 Experiment Drive, Hwy. 182 Jay, FL 32565 (850) 995-3720 x108, jbu@ufl.edu This project and the preparation of this report were funded by a grant from the Florida Department of Environmental Protection (FDEP).
Project Title: Warm-Season Turfgrass N Rates & Irrigation BMP Verification Project 2: Effect of N Rate on Nitrate-N Leaching in Established Grasses EXECUTIVE SUMMARY Nitrate-N leaching varied due to location in 2 out of 3 yrs and in the total of the 3-yr period. In these cases, differences appeared not to be related to soil type or season, but rather to extensive winterkill injury in northwest FL. There were minimal differences due to nitrogen rate (NR); when there were differences, nitrate-n leaching was greater at the higher NRs. There were no differences due to irrigation. Increased percent of applied N leached was attributed to a number of high rainfall events in Jay in yr 1. Excluding these data, percent of applied nitrogen (N) leached ranged from 0.2 to 4.4%, with higher percentages at the low NRs, which is typical of published reports. Maintaining a dense, healthy turf is an important factor in reducing nitrate-n leaching and it is important that proper turf management reflect this.
Introduction Irrigation and nitrogen fertilization are essential components of turfgrass maintenance programs that provide healthy, good quality St. Augustinegrass lawns. Applied at the appropriate rates as suggested by current IFAS guidelines, N and irrigation have been shown to improve turfgrass quality, color, health and vigor. However, excess N from over-fertilization in combination with over-irrigation can potentially increase nitrate-n leaching and contribute to degradation of ground and surface water quality. There are currently increased restrictions on fertilization of home lawns due to these concerns, but the precise nitrate-n loading that might be expected from fertilization of a lawn grass has not been precisely quantified. Nor is there relevant published literature available on the quantity of N or irrigation needed to produce acceptable turfgrass quality and health for home lawns throughout Florida. The objective of this experiment was to determine the effects of two irrigation regimes and four N rates (ranging from sub-optimal to excess fertilization) on N leaching from St. Augustinegrass at each of three geographic locations in Florida. The information provided from this research will help in the development of Best Management Practices (BMPs) that improve water quality and conservation while providing healthy turf systems. Materials and Methods The research was conducted at the Ft. Lauderdale Research and Education Center, the Plant Science Research and Education Unit in Citra, and the West Florida Research and Education Center in Jay. St. Augustinegrass (Stenotaphrum secundatum Walt. Kuntze) cv. Floratam was planted in each location and allowed to establish prior to initiation of treatments. Plots were 4 m x 4 m with a lysimeter in the center of each plot as previously described (Trenholm et al., 2009, 2008, 2007). Nitrogen rates for each location are listed in Table 1 and were designed to provide suboptimal, medium, and excess rates for each of the three locations, based on existing IFAS recommendations for fertilization of St. Augustinegrass. Treatment application in Ft Lauderdale began 12 October, 2006 and continued every other month after that for a total of 12 fertilizer cycles. Main blocks (8 x 4 m) consisted of one of two irrigation regimes: 2.5 mm daily (Low) except when daily precipitation > 6.4 mm (irrigation turned off), and 13 mm three times weekly (High). In Citra, sod was established in spring of 2005 and treatments began in July 2005. There were 2 treatment applications in 2005 and 4 each in 2006 and 2007 for a total of 10 fertilizer cycles. Irrigation treatments consisted of 13 mm applied twice weekly or 25 mm applied weekly. When rainfall met or exceeded these amounts, irrigation was suspended. In Jay, fertilizer and irrigation treatments began in summer 2005 through 2007 for a total of 10 fertilizer cycles. Irrigation treatments consisted of 13 mm applied twice weekly or 25 mm applied weekly. In all locations, the N source used was soluble urea (46-0-0) and applied as a spray for all treatment applications. Irrigation regimes were based on local estimates of evapotranspiration (ET).
Lysimeters were evacuated and samples taken as previously described (Trenholm et al., 2009, 2008, 2007) twice a week during this study period. Any nitrate-n concentration that was lower than the minimum detection limit (MDL) of 0.05 mg L -1 was corrected to the MDL value. Visual turf ratings, closely linked to turf health and vigor, were based on a scale of 1-9 (9 = healthy, growing uniform turf, 1 = dead turf and 6 = minimally acceptable turf) and were made bi-weekly. Experimental design in each location was a split plot with 4 replications. Main plots were irrigation regime and sub plots were N rate. Following correction for outliers, all data were analyzed with PROC GLM or PROC REG (SAS Institute, 1999) Significance was determined at the 5% probability level. To compare data between the three locations, 8 fertilizer cycles were analyzed over a 3-yr period. Total nitrate-n leaching data are reported for the 3-yr period and annually and percent of applied N leached is reported by year. Results Hydrology Rainfall data are displayed in Fig 1-3 for each of the locations over the trial period. Events that produced greater than 25 mm in a day are considered excessive. Total Nitrate-N Leached Amount of nitrate-n leached differed due to the interaction of location (L) * nitrogen rate (NR) in yr 1 and by L in yr 1 and 3 and when totaled over the 3 yrs. In yr 2, there was a difference due to NR. There were no responses to irrigation (I). Means of nitrate-n leaching values for all locations by NR and year are shown in Table 3. Nitrate-N leaching was greater in Jay than in the other locations in yr 3 and for the total of yrs 1-3 (Table 4). This increased leaching at Jay in yr 3 is related to the decreased turf quality and density in response to winterkill from the winter months preceding yr. 3 and rainfall totaling 536 mm. Turf that is not healthy and is not growing or providing adequate root and shoot cover results in greater potential for nitrate-n leaching because it has less probability to intercept the fertilizer. The ability of damaged turf to assimilate N is not as well documented as other factors related to nitrate-n leaching, but similar findings were reported in Floratam that had sustained loss of density due to insect damage in a study conducted in tubs in a greenhouse. Sharma et al. (2009) reported that nitrate-n leaching from grass with insect damage did not exceed 0.7 % of the applied N, but was higher than from previous cycles when the grass was not damaged and provided better cover. The interaction of L*NR in yr 1 is shown in Table 5. There were no significant differences within locations for nitrate-n leaching in response to NR for this year. There
was significantly greater nitrate-n leaching in Jay compared to the other locations, which may be due to excessive rainfall received during the first year of research. There was a response to the main effect of NR averaged across locations only in yr 2 (Table 6). Highest nitrate-n leached was from grass that received 490 kg N ha -1 (Citra location only), with no differences in leaching from the other NRs. It is not surprising that there would be significant nitrate leaching in Citra from this NR, which is 10 times higher than the highest currently recommended rate and 20 times greater than the currently recommended application for water soluble N. There were no differences in response to NR for nitrate-n leached in Ft. Lauderdale or Jay for yr. 2. There were no other significant responses to NR throughout this study period. Previous research has documented the ability for uptake of nitrogen by St. Augustinegrass. In a greenhouse study, Bowman et al. (2002) reported volume-weighted nitrate-n totals of 0.9 to 8.0 mg L -1 lost from Raleigh St. Augustinegrass and Meyer zoysiagrass (Zoysia japonica Steud), respectively. Percentages of applied N leached ranged form 0.9 to 17.6% in Raleigh and Meyer, respectively. Erickson et al. (2001) reported annual inorganic-n loss of 4.1 kg ha -1 from Floratam and concluded that St. Augustinegrass was a more effective filter for leachate reduction than a mixed landscape planting. Trenholm et al. (2008, 2008, 2007) reported higher nitrate-n leaching from bahiagrass (Paspalum notatum Flugge), centipedegrass (Eremochlua ophiuroides [Munro] Hack.), and Empire zoysiagrass in comparison to Floratam in studies throughout Florida. Previous research on other turf species has documented the effects of NR on nitrate-n leaching. Frank et al. (2006) reported higher total nitrate-n losses when N was applied at annual rates of 245 rather than 98 kg N ha -1 to Kentucky bluegrass (Poa pratensis L.). The authors concluded that application of the low NR provided minimal potential for groundwater pollution, but that the high rate, particularly when applied as a single dose, water soluble N source, may result in nitrate N levels in excess of the USEPA safe levels of 10 mg nitrate-n L -1. Morton et al. (1988) reported greatest annual flow-weighted nitrate concentration (4.02 mg L -1 ) in Kentucky bluegrass that received high N (244 kg N ha -1 yr -1 ) and excessive irrigation (3.75 cm wk -1 regardless of rainfall), well below the USEPA safe water standards. Annual nitrate losses for this treatment totaled 32 kg nitrate-n ha -1. The authors concluded that inorganic N leaching losses from appropriate home lawn care practices would not contribute to ground water contamination, but that care should be used when fertilizing lawns in coastal watersheds. Single degree of freedom contrasts between NRs are displayed in Table 2. In yr 1 and over the 3-yr study period, there were no differences in nitrate-n leaching between 196 kg N ha -1 (common to all 3 locations and part of IFAS current fertilizer recommendations for all 3 geographical locations) and all other NRs. In yr.2, there was a significant difference between 196 and 490 kg N ha -1. This is associated with the higher nitrate leached at Citra in this year at the 490 kg ha -1 NR, as previously discussed. The 490 kg ha -1 rate did not differ from either the 98 or the 343 kg ha -1 NR. In yr 3, there was a contrast between 196 and 49 kg ha -1, averaged over all Jay and Citra locations. Although analysis of variance did not detect differences in leaching between any rates in either
Citra or Jay in this year, there was approximately a 53% reduction in nitrate-n leaching at Jay from plots that received N at 49 vs. those that received 196 kg ha -1. This increased leaching at Jay in yr 3 is related to the decreased turf density in response to winterkill from the previous year. Sharma et al. (2009) reported a similar finding in nitrate-n leaching from Floratam that had sustained loss of density due to insect damage in a study conducted in tubs in a greenhouse. Nitrate-N leaching in this study, even with insect damage, did not exceed 0.7 % of the applied N. In addition to NR, other factors have also been studied with respect to their influence on nitrate-n leaching. The influence of NR and irrigation, while not significant in this research, has been shown to strongly influence nitrate-n movement in some published reports. Snyder et al. (1984) reported significantly less nitrate-n leached from bermudagrass (Cynodon dactylon L. x C. transvaalensis Burtt-Davy) that received sensor-based irrigation treatments than from plots receiving a daily irrigation regime. Brown et al. (1977), also working with bermudagrass, reported that nitrate-n losses were a function of NR and applied irrigation, with minimal losses from inorganic N sources when irrigation was matched with evapotranspiration (ET). Clearly, proper irrigation management is critical for both reduction of potential leaching losses and for maintenance of a healthy turf. Other factors reported to influence nitrate-n leaching include N source (Easton and Petrovic, 2004; Guillard and Kopp, 2004; Saha et al., 2007). Some published reports have noted differences in nitrate-n leached in response to N source, while others report few differences. We chose a highly soluble N source for this research to represent a worstcase scenario in representing nitrate-n leaching. Maturity of the grass has been noted by numerous researchers as impacting nitrate-n leaching (Bowman et al., 2002; Easton and Petrovic, 2004; Erickson et al., 2001; Geron et al., 1993), as well as root architecture (Bowman et al., 1998). Nitrate-N levels reported here are much lower than those reported on newly established turf (Trenholm et al., 2009, 2008, 2007) due to the ability of St. Augustinegrass to provide dense root and shoot cover when established. Percent of Applied N Leached The percent of applied N that leached differed due to an interaction of L*NR in yr 1 only and due to main effect of NR in yr 2 (Table 2). In yr 1, Ft. Lauderdale observed no response to NR, while in both Citra and Jay, highest percentage leached was from turf that received the lowest NR of 49 kg ha -1 (Table 5). This is a typical response from lower NRs, as low levels of applied NO 3 - -N can result in high percentages leached given the small amount applied. In yr. 2, there was a response to NR across locations. The low rate of 49 kg ha -1 resulted in the highest percentage of leached nitrate-n (Table 6). Percentages at all other NRs were below 1% of applied N. There were no responses to irrigation in any year. Regression Analysis
Regression analysis over the 3 locations showed no association between NR and nitrate- N leached (Table 7) in any of the 3 yrs, as demonstrated by the r 2 values. This means that, even at the range of NRs applied in each location, nitrate-n leaching did not increase in a linear fashion with NR. The authors are not aware of any published reports demonstrating such a response in nitrate-n leaching from a turf area. Turf Visual Ratings In Ft. Lauderdale, turfgrass visual ratings were significantly affected by rate of N applied in all years (Table 8). In yr 1 and 2, turf visuals were reduced below the minimal acceptable level on some rating dates at the lowest NR. In yr 3, quality scores remained above the acceptable level at all NRs. Irrigation rate had no effect on St. Augustinegrass quality in any year. In Citra, grass that received N at 49 kg ha -1 had lower than acceptable visual ratings for yrs 2 and 3 for each rating period throughout each season. For conditions upon which this research was conducted, this NR was not sufficient to maintain the grass in a healthy condition. While this NR did not result in increased nitrate-n leaching, a majority of homeowners would be dissatisfied with the vigor, density, color, and uniformity of a lawn fertilized at this rate. There was no response to irrigation for turf visual ratings. In Jay, all plots had above acceptable visual ratings in yr 1, with increased visual scores in response to NR on approximately half of the evaluation dates and when averaged over the season. In yr 2, all evaluation dates had increasing visual scores in response to NR. By yr 3, due to winterkill from the 2007 winter season, all visual and density scores were below acceptable until mid July, with no difference in response due to NR. As the turf improved over the summer, visual scores and density increased in response to NR. The high nitrate-n leaching observed in yr 3 was directly related to the sub-standard turf health and density. Poor quality turf has a significantly reduced ability to take up nitrogen; therefore maintenance of a healthy, growing turf (acceptable standards) is an important component of water quality. Conclusions Results of this research indicate that healthy, established St. Augustinegrass leaches low levels of nitrate-n. On a statewide basis, nitrate-n leaching had few responses to NR in Floratam St. Augustinegrass. Exceptions to this were found in turf that had suffered damage due to winterkill in Jay, due to reduced root and shoot growth and ability to intercept the fertilizer. Regression analysis likewise showed no association with applied NR and nitrate-n leached. Over the 3-yr study period, these results indicate that water soluble nitrogen could be applied at higher than currently allowed rates to healthy turfgrass without increased leaching of nitrate-n. In spite of the documented ability for N uptake demonstrated by St. Augustinegrass, there are many other factors that may also contribute
to nitrate-n leaching Due to these other factors and due to the complex dynamics of N cycling in turf, it is recommended that the N application rates currently recommended by IFAS be maintained, although the single application amounts for water soluble N sources could be reviewed. These rates supplied acceptable turf density and supported a healthy stand of grass with minimal nitrate-n leaching. For optimal reduction of nitrate-n movement from lawns, it is critical to address proper cultural practices, including irrigation and mowing height. Presence of biotic and abiotic stresses must also be considered, as these can lead to reduced cover and increased potential for nitrate-n leaching. References Bowman, D.C., C.T. Cherney, and T.W. Rufty, Jr. 2002. Fate and transport of nitrogen applied to six warm-season turfgrasses. Crop Sci. 42:833-841. Bowman, D.C., D.A. Devitt, M.C. Engelke, and T.W. Rufty, Jr. 1998. Root architecture affects nitrate leaching from bentgrass turf. Crop Sci. 38:1633-1639. Brown, K.W., R.L. Duble, and J.C. Thomas. 1977. Influence of management and season on fate of N applied to golf greens. Agron. J. 69:667-671. Easton, Z.M. and A.M. Petrovic. 2004. Fertilizer source effect on ground and surface water quality in drainage from turfgrass. J. Environ. Qual. 33:645-655. Erickson, J.E., J.L Cisar, J.C. Volin, and G.H. Snyder. 2001. Comparing nitrogen runoff and leaching and between newly established St. Augustinegrass turf and an alternative residential landscape. Crop Sci. 41:1889-1895. Frank, K.W., K.M O Reilly, J.R. Crum, and R.N. Calhoun. 2006. The fate of nitrogen applied to a mature Kentucky bluegrass turf. Crop Sci. 46:209-215. Geron, C.A., T.K. Danneberger, S.J. Traina, T.J. Logan, and J.R. Street. 1993. The effects of establishment methods and fertilization practices on nitrate leaching from turfgrass. J. Environ. Qual. 22:119-125. Guillard, K. and K.L. Kopp. 2004. Nitrogen fertilizer form and associated nitrate leaching from cool-season lawn turf. J. Environ. Qual. 33:1822-1827. Morton, T.G., A.J. Gold, and W.M. Sullivan. 1988. Influence of overwatering and fertilization on nitrogen losses from home lawns. J. Environ. Qual. 17:124-130. Saha, S.K., L.E. Trenholm, and J.B. Unruh. 2007. Effect of fertilizer source on nitrate leaching and St. Augustinegrass turfgrass quality. Hort Sci. 42:1478-1481. SAS Institute, 2003. SAS/STAT user s guide. Version 8. SAS Inst., Cary, NC. Sharma, S., L. E. Trenholm, J.B. Unruh, and J.B. Sartain. 2009. Effect of fertilizer rates and mowing heights on nitrate leaching from St. Augustinegrass. Proc. Fla. State Hort. Soc. In press. Snyder, G.H., B.J. Augustin, and J.M. Davidson. 1984. Moisture sensor-controlled irrigation for reducing N leaching in bermudagrass turf. Agron. J. 76:964-969. Trenholm L.E., J.L. Cisar, J.B. Sartain, and J.B. Unruh. 2009. Warm-season turfgrass N rates and irrigation BMP verification. Year 3 annual report to Florida Dept. of Environmental Protection. Trenholm L.E., J.L. Cisar, J.B. Sartain, and J.B. Unruh. 2008. Warm-season turfgrass N
rates and irrigation BMP verification. Year 2 annual report to Florida Dept. of Environmental Protection. Trenholm L.E., J.L. Cisar, J.B. Sartain, and J.B. Unruh. 2007. Warm-season turfgrass N rates and irrigation BMP verification. Year 1 annual report to Florida Dept. of Environmental Protection.
Table 1. Treatment annual rates by location. Location Annual N Rates Ft. Lauderdale 98 196 294 588 Citra 49 196 343 490 Jay 49 98 196 294 Table 2. Analysis of variance of main effects and interactions on nitrate-n leaching and single degree of freedom contrasts between NRs. Source of variation TN Yr1 TN Yr2 TN Yr3 TN Yr1-3 PC N Yr1 PCN Yr2 PCN Yr3 Location (L) *** NS ** ** ** NS NS Nitrogen Rate (NR) NS ** NS NS *** *** NS L*NR * NS NS NS *** NS NS Irrigation (I) NS NS NS NS NS NS NS L*I NS NS NS NS NS NS NS NR*I NS NS NS NS NS NS NS L*NR*I NS NS NS NS NS NS NS Contrasts - NR 196 vs 49 kg ha -1 NS NS * NS 196 vs 98 kg ha -1 NS NS NS NS 196 vs 294 kg ha -1 NS NS NS NS 96 vs 343 kg ha -1 NS NS NS NS 196 vs 490 kg ha -1 NS *** NS NS 196 vs 588 kg ha -1 NS NS NS NS TN= Total nitrate-n leached. PC N = Percent of applied N leached. ***,**,* Significant at P = 0.001, 0.01, and 0.05 levels, respectively. NS = not significant.
Table 3. Means of nitrate- N leaching values and percent of applied N leached at 3 locations in response to NR. Means are averages of 4 replications per location over a 3-yr period. Location NR Nitrate-N Leaching Percent of Applied N Leached 3-yr Yr 1 Yr 2 Yr 3 Yr 1 Yr 2 Yr 3 Ft L 98 2.0 0.3 0.5 1.1 0.9 1.1 2.3 196 2.7 1.1 0.5 1.1 1.7 0.5 1.1 294 1.7 0.3 0.5 1.0 0.3 0.3 0.7 588 3.2 1.7 0.6 0.9 0.9 0.2 0.3 Citra 49 2.3 1.4 0.4 0.4 4.4 1.2 1.1 196 3.0 1.9 0.4 0.7 1.4 0.3 0.5 343 3.9 1.8 1.3 0.8 0.8 0.5 0.3 490 5.3 2.4 2.3 0.6 0.7 0.6 0.2 Jay 49 18.8 9.7 0.7 8.4 29.7 1.4 17.2 98 17.3 8.4 0.5 8.4 12.8 0.5 8.6 196 23.7 4.8 0.7 18.2 3.7 0.4 9.3 294 35.2 11.6 0.7 22.9 5.9 0.2 0.2 Table 4. Nitrate-N leaching due to location in yr 3 and 1-3. Nitrate-N Leaching Yr 3 Yr 1-3 Ft. Lauderdale 1.0 b 2.4 b Citra 0.6 b 3.6 b Jay 14.5 a 23.8 a Means followed by the same letter within a column do not differ at the 0.05 probability level.
Table 5. Location (L) * nitrogen rate (NR) interaction for total nitrate-n leached in yr 1 and over the 3-yr study period and percent of applied N leached in yr 1. Nitrogen Rate Location 49 98 196 294 343 490 588 P value Year 1 Nitrate-N Leached Ft L 0.3 1.1 0.3 1.7 NS Citra 1.4 1.9 1.8 2.4 NS Jay 9.7 8.4 4.8 11.6 NS Year 1 Percent of Applied N Leached Ft L 0.9 1.7 0.3 0.9 NS Citra 4.4 a 1.4 b 0.8 b 0.7 b * Jay 29.7 a 12.8 b 3.7 b 5.9 b ** *,** Significant at P = 0.01, and 0.05 levels, respectively, within a row. NS = not significant within a row. Table 6. Nitrogen rate (NR) response for total nitrate-n and percent of applied N leached averaged across locations in yr 2. Nitrogen Rate Nitrate-N Leached Percent of Applied N Leached 588 0.6 b 0.2 d 490 2.3 a 0.6 bc 343 1.3 b 0.5 cd 294 0.6 b 0.3 cd 196 0.6 b 0.4 cd 98 0.5 b 0.8 b 49 0.6 b 1.3 a Means followed by the same letter within a column do not differ at the 0.05 probability level. Table 7. Coefficient estimates of nitrate-n leaching across 3 locations. Nitrate-N Leaching β 0 β 1 Β 2 r 2 P Year 1 5.1-0.27 NS 0.03 NS Year 2 0.4 0.08 NS 0.05 * Year 3 0.3 0.05 NS 0.06 ** Year 1-3 5.8-0.14 NS 0.0009 NS **,* Significant at P = 0.01, and 0.05 levels, respectively, within a row. NS = not significant.
Table 8. Analysis of variance for turf visual scores. Ft. Lauderdale Citra Jay Source of Yr 1 Yr 2 Yr 3 Yr 1 Yr 2 Yr 3 Yr 1 Yr 2 Yr 3 Variation NR * * ** ** ** *** * * * I NS NS NS NS NS NS NS NS NS NR*I NS NS NS NS NS 0.001 NS NS NS ***,**, * Significant at P = 0.001, 0.01, and 0.05 levels, respectively, within a column. NS = not significant.
Fort Lauderdale Daily Rainfall (mm) 100 80 60 40 20 0 mm 1 Oct 06 1 Dec 06 1 Feb 07 1 Apr 07 1 Jun 07 1 Aug 07 1 Oct 07 1 Dec 07 1 Feb 08 1 Apr 08 1 Jun 08 1 Aug 08 1 Oct 08 1 Dec 08 Fig. 1 Daily rainfall (mm) in Ft. Lauderdale during the study period.
80 70 60 50 40 30 20 10 0 Project 2 Daily Rainfall (mm) Rainfall (mm) 24 Jul 05 7 Aug 05 21 Aug 05 4 Sep 05 18 Sep 05 2 Oct 05 16 Oct 05 30 Oct 05 13 Nov 05 6 Apr 06 20 Apr 06 4 May 06 18 May 06 1 Jun 06 15 Jun 06 29 Jun 06 13 Jul 06 27 Jul 06 10 Aug 06 24 Aug 06 7 Sep 06 21 Sep 06 5 Oct 06 19 Oct 06 2 Nov 06 2 Apr 07 16 Apr 07 30 Apr 07 14 May 07 28 May 07 11 Jun 07 25 Jun 07 9 Jul 07 23 Jul 07 6 Aug 07 20 Aug 07 3 Sep 07 17 Sep 07 1 Oct 07 15 Oct 07 29 Oct 07 12 Nov 07 Fig. 2. Fig. 1 Daily rainfall (mm) in Citra during the study period.
1 Jul 07 1 Sep 07 Jay Daily Rainfall (mm) 140 120 100 80 60 40 20 0 1 Jul 05 1 Sep 05 1 Nov 05 1 Jan 06 1 Mar 06 1 May 06 1 Jul 06 1 Sep 06 1 Nov 06 1 Jan 07 1 Mar 07 1 May 07 Fig. 3. Daily rainfall (mm) in Jay during the study period. mm