﻿
 UFDC Home myUFDC Home  |   Help  |   RSS
<%BANNER%>

 Calculating Nutrient Loads http://edis.ifas.ufl.edu/ ( Publisher's URL )
CITATION PDF VIEWER
Full Citation
STANDARD VIEW MARC VIEW
 Material Information Title: Calculating Nutrient Loads Physical Description: Fact Sheet Creator: Rice, Ronald W. Publisher: University of Florida Cooperative Extension Service, Institute of Food and Agriculture Sciences, EDIS Place of Publication: Gainesville, Fla. Publication Date: 2001
 Notes Acquisition: Collected for University of Florida's Institutional Repository by the UFIR Self-Submittal tool. Submitted by Melanie Mercer. Publication Status: Published General Note: "First Published: August 1998; Revised September 2001." General Note: "Bulletin 906"
 Record Information Source Institution: University of Florida Institutional Repository Holding Location: University of Florida Rights Management: All rights reserved by the submitter. System ID: IR00001511:00001

AE14900 ( PDF )

Full Text

PAGE 1

PAGE 2

PAGE 3

Calculating Nutrient Loads 3 would be (53 in)*(1 ft/12 in)*(the EAA area in ft2). Likewise, expressing drainage as a single dimensioned value (i.e. "farms in the EAA drain about 11 in of water annually") is actually a volume of water (in3) divided by the area (in2) being referred to. 4. Cubic foot (ft3) Arguably the most commonly used English unit dimension for expressing large volumes of water, equivalent to the amount of water that would fit in a cube with 1 ft long sides. (7.481 gal/ft3 or 28.31559 L/ft3). 5. Gallon (gal) Arguably the most common everyday English unit representation of a liquid volume. There are 7.481 gal of water in 1 ft3. (3.785 L/gal). 6. Liter (L) Arguably the most common everyday metric unit representation of a liquid volume. There are 1000 milliliters (mL) of water in a L. (0.2642 gal/L or 0.03532 ft3/L). 7. Cubic meter (m3) Arguably the most common metric unit representation of large liquid volumes. A cubic meter (m3) of water is the volume of water that will fit in a cube with sides 1 m in length. There are 1000 L in a m3. 8. Cubic centimeters (cm3 or cc) A metric unit representation of small liquid volumes, equivalent to the amount of water that will fit in a cube with sides 1 cm in length. There are 1000 cm3 in a L. 9. Milliliter (mL) A metric unit volume equal to 1/1000 of a liter. A mL is a volume equivalent to a cm3. 10. Centimeter or Meter (cm or m) The metric units of length used to express a volume of water as is done with the English units in and ft. Mass or Weight 1. Pound (lb) The most common English unit for mass or weight. (0.4536 kg/lb). There are 2000 lb in a ton. A gal of water is generally estimated to weigh 8.33 lb. 2. Kilogram (kg) Arguably the most common metric unit representation of mass or weight. There are 1000 kg in a metric ton (MT) and 1000 grams in a kg. A kg is equivalent to approximately 2.2046 lb. 3. Gram (g) A metric unit of mass equal to 1/1000 of a kg. There are approximately 453.6 g in a lb. 4. Milligram (mg) A metric unit of mass equal to 1/1000 of a gram or 1/1,000,000 of a kg. 5. Microgram ( g) A metric unit of mass equal to 1/1,000 of a mg, 1/1,000,000 of a g, or 1/1,000,000,000 of a kg. Concentration 1. Milligram per liter (mg/L) The most common metric unit describing a mass of chemical present in a L of water (mass/volume). A mg/L is equivalent to the English unit "parts per million" (ppm) when the assumption that 1 mL of water weighs 1 g (density of water is 1 g/mL, cc, or cm3) holds true. Therefore, 1 mg/L is the same as saying a mg of chemical per 1,000 mg of water (mass/mass). Small variations in the density of water occur due to atmospheric pressures, temperatures, and water purity. However, for general purposes the variations cause inconsequential differences. 2. Microgram per liter (g/L) A metric concentration unit equivalent to 1/1,000 of a mg/L. A g/L is equivalent to the English unit "parts per billion" (ppb) subject to the assumptions described above. A g/L is a mass/volume unit, and is numerically equivalent to a mass/mass unit since the density of water in metric units is one g/cm3. 3. Parts per million (ppm) A dimensionless English unit expression of the mass of a chemical in a mass of water. For example, the concentration of a chemical in water may be expressed as 1 lb of chemical in 1,000,000 lb of water. Here again, the density of water is assumed to be a constant although it changes slightly with changes in atmospheric pressure, temperature, and water purity. A ppm is equal to

PAGE 4

PAGE 5

Calculating Nutrient Loads 5 flows and P concentrations leaving farms in the EAA during pumped drainage events in response to rainfall. Fig 1. Typical phosphorus flow rate curve showing changes over time. Fig 2. Typical phosphorus concentration curve showing changes over time, typical of an event with a "first-flush." tt The flow rate curve in Figure 1 shows a slowly declining flow rate over time as hydraulic heads in the system increase. Over time, as the EAA main canal levels rise, and farm canal levels drop, pumps must work harder to lift water from a falling water level within the intake sump (on-farm) and discharge it to a sump or canal (off-farm) where water levels are rising. As the discharge event progresses, this hydraulic head that the pump must work against increases and pump efficiencies decline, yielding an accelerated decrease in flow rate towards the end of the pumping event. The concentration curve in Figure 2 shows an initially high P concentration, typical of a "first-flush" type event, where large amounts of P-bearing particulate matter and sediments near the pump intake are initially discharged. As the event continues, concentrations decline as rain water and open canal/ditch water P species dominate the P concentration characteristics of the water in the drainage stream. Another typical P concentration distribution is shown in Figure 3. In this case, the drainage water concentration starts low, indicative of a situation where there is little particulate matter transport or channel bottom sediment scouring when the pump starts up. Farm canal water dissolved-P concentrations could also be relatively low. Events that could cause this phenomenon are dilution of the farm canal P concentrations due to initially heavy rainfall, antecedent pumping where much of the particulate matter or sediment near the pump had already been moved during the preceding pumping event, area main canal leakage through the pump station into the farm canal, seepage into the farm canal through the fractured bedrock, irrigation occurring just prior to drainage pumping, and/or a slow initial pump speed. As the event progresses, P concentrations rise. This could be attributed to the transport of particulate matter and sediments from the downstream reaches of the farm canal, lower water levels accompanied by higher flow velocities and canal bottom scouring, increased pump speed, and/or the addition to the drainage stream of rain water which has fallen on the field surfaces and passed through, or over, the P-rich upper soil layer carrying mobile particulates and dissolved-P.

PAGE 6

Calculating Nutrient Loads 6 Fig 3. Phosphorus concentration change over time typical of an event where concentrations increase as soil water and particulate matter enter the drainage stream late in the event. Shown in Figure 4 is another typical P distribution found in the EAA. In this case, concentrations start low, peak sometime during the event, and then fall off later in the event. This distribution can generally be attributed to a different time distribution of the occurrences discussed above that cause the distributions shown in Figure 1 and Figure 2. Fig 4. Phosphorus concentration change over time typical of an event where concentrations increase as soil water and particulate matter enter the drainage stream in the middle of the event before declining. For additional information on water sampling strategies and load calculations applicable to the EAA, the reader is referred to Izuno, Bottcher, and Davis (1992), Izuno et al. (1996), Izuno and Rice, eds. (1997), and Rice and Izuno (1997). Calculating Nutrient Loads For Typical Data Collection Protocols Beginning and End of Event Sampling Shown in Figure 5 and Figure 6 are data derived from one of the most rudimentary sampling protocols acceptable for nutrient load determinations. In this case, water samples are collected at the beginning and end of the drainage event. Flow rates are also calculated only for the beginning and end of the event, usually by collecting hydraulic head and pump rpm data when turning the pump(s) on and off and applying a pump calibration equation. To calculate the nutrient load using these available data, the beginning and end concentrations and flow rates can simply be averaged. The average flow rate is then multiplied by the length of time that the pump ran (pumping duration) to determine the total volume of water discharged. The equation to apply to calculate load with these data, using the

PAGE 7

Calculating Nutrient Loads 7 Fig 5. Typical phosphorus concentration curve showing changes over time when data are collected only at the beginning and end of the pumping event. Fig 6. Typical phosphorus flow rate curve showing changes over time when data are collected only at the beginning and end of the pumping event. variable names assigned in Figure 5 and Figure 6, is Equation 2: Equation 2. where C1 and C2 are the beginning and end of event concentrations, respectively, a and b are the beginning and end of event flow rates, respectively, and Time is the pumping duration. Looking back at Figure 1, Figure 2, Figure 3, and Figure 4, one can see that this equation would yield a fairly representative load for the event, except when the concentration distribution depicted in Figure 4 applies. Flow rates in the EAA are generally flat over the normal pump operating range. Hence, an event average flow rate calculated as in Equation 2 is probably sufficient unless head differences across the pump change greatly during the event, or the pump is not installed to operate under its recommended efficient operating range. Time Discrete Water Sampling With Continuous Flow Monitoring At times it is desirable to know the distribution characteristics of drainage water P concentrations over time for the purpose of identifying factors which cause the distribution to appear as it does. In this case, time discrete water sampling (collecting water samples on predetermined, closely spaced, time intervals) is useful. Concentration data typical of the above water sampling protocol are shown in Figure 7. Again, Figure 8 again shows the typical continuously monitored flow rate versus time curve. In Figure 7, it is evident that drainage water sample concentrations have been determined on two-hour intervals. Load calculations are still relatively simple. However, one must consider

PAGE 8

Calculating Nutrient Loads 8 Fig 7. Phosphorus concentration distribution through a drainage event with water samples collected every two hours. Fig 8. Flow rate distribution through a drainage event with flows calculated every two hours. several calculation options, select one, and apply the selected option consistently. The primary determination that one must make is "what volume of pumped water corresponds to each concentration value". Options are: 1) Use the two-hour flow time period following the concentration time; 2) Use the two-hour flow time period preceding the concentration time; 3) Average two adjacent concentrations to obtain an average concentration for each two-hour flow period; or 4) Average the flow rates measured an hour before and an hour after the concentration was measured. For time discrete water sampling (Figure 7), where a water sample represents a point-in-time measurement, option 4 is the most desirable. The representative flow rate is then multiplied by the time interval to determine flow volume, and load is then calculated. In this example, it is assumed that each concentration data point is a time discrete measurement representing the drainage stream concentration at a point-in-time. Hence, one must infer that the concentration distribution is a smooth curve (Figure 7). The load calculation option of choice is to select the flow rates at appropriate times and use them to calculate the flow volume. Illustrating this example, Figure 7 shows a concentration C at time=14 hours. Water samples were collected every two hours. Hence, the sample concentration should be assumed to apply to the volume of water pumped an hour before and an hour after the time that the water sample was collected (half the time step before plus half the time step after the time that the water sample was collected). In Figure 8, the flow rates at time=13 hours and time=15 hours are different, and should be averaged and multiplied by two hours to calculate the flow volume for concentration C. This flow volume is represented by the shaded area in Figure 8. Since there are 10 discrete water sample concentrations, the total event load would then be the summation of all the incremental loads associated with each two-hour time period as written in Equation 3: Equation 3.

PAGE 9

Calculating Nutrient Loads 9 where load0 is calculated using the flow volume that occurred during the first hour after pump start-up, load10 is calculated using the flow volume that occurred during the last hour prior to pump shut-down, and loads1-9 are calculated using the two-hour flow volumes (the total volume pumped starting one hour preceding water sample collection and ending one hour after sample collection). The calculation for the example in Figure 7 and Figure 8 is shown in Equation 4: Equation 4. where Timec=13 hours, Timed=15 hours, a=flow rate at 13 hours, and b=flow rate at 15 hours, C14=water sample concentration at 14 hours, and Load14=two-hour incremental load associated with C14. Flow Composite Discrete Sampling With Continuous Flow Monitoring Shown in Figure 9 is a typical flow-weighted P concentration distribution where water samples are collected several times during an event, based on the pumping of a predetermined volume of water. In other words, each water sample is collected after a predetermined and equal volume of water is pumped. Hence, each concentration value is representative of an equal volume of water. This differs from time discrete water sampling (discussed above) where water samples were collected at predetermined time intervals, regardless of actual flow conditions. In this example, the "equal volume of water pumped" is shown as the shaded areas "a" in Figure 10. The flow volumes between water sample concentrations used in the load equation are fixed, equal, and predetermined. Now, it becomes simply a matter of multiplying that fixed volume by each concentration and summing the incremental loads (Equation 5): Fig 9. Flow-weighted phosphorus concentrations. Fig 10. Predetermined equal flow volumes between hours 0 and 20.

PAGE 10