Continuous Instream Monitoring of Nutrients and Sediment in Agricultural Watersheds

Summary

With the advancement of technology and the rise in end-user expectations, the need and use of higher temporal resolution data for pollutant load estimation has increased. This protocol describes a method for continuous in situ water quality monitoring to obtain higher temporal resolution data for informed water resource management decisions.

Abstract

Pollutant concentrations and loads in watersheds vary considerably with time and space. Accurate and timely information on the magnitude of pollutants in water resources is a prerequisite for understanding the drivers of the pollutant loads and for making informed water resource management decisions. The commonly used "grab sampling" method provides the concentrations of pollutants at the time of sampling (i.e., a snapshot concentration) and may under- or overpredict the pollutant concentrations and loads. Continuous monitoring of nutrients and sediment has recently received more attention due to advances in computing, sensing technology, and storage devices. This protocol demonstrates the use of sensors, sondes, and instrumentation to continuously monitor in situ nitrate, ammonium, turbidity, pH, conductivity, temperature, and dissolved oxygen (DO) and to calculate the loads from two streams (ditches) in two agricultural watersheds. With the proper calibration, maintenance, and operation of sensors and sondes, good water quality data can be obtained by overcoming challenging conditions such as fouling and debris buildup. The method can also be used in watersheds of various sizes and characterized by agricultural, forested, and/or urban land.

Introduction

Water quality monitoring provides information on the concentrations of pollutants at different spatial scales, depending upon the size of the contributing area, which can range from a plot or a field to a watershed. This monitoring takes place over a period of time, such as a single event, a day, a season, or a year. The information garnered from monitoring water quality, mainly relating to nutrients (e.g., nitrogen and phosphorus) and sediment, can be used to: 1) understand hydrological processes and the transport and transformation of pollutants in streams, such as agricultural drainage ditches; 2) evaluate the efficiency of management practices applied to the watershed to reduce the nutrient and sediment load and to increase the water quality; 3) assess the delivery of the sediment and nutrients to the water downstream; and 4) improve the modeling of nutrients and sediment to understand the hydrological and water quality processes that determine pollutant transport and dynamics over the range of temporal and spatial scales.

This information is crucial to aquatic ecosystem restoration, sustainable planning, and the management of water resources¹.

The most commonly used method for nutrient and sediment monitoring in a watershed is grab sampling. Grab sampling accurately represents a snapshot concentration at the time of sampling². It can also depict a variation of pollutant concentrations with time if frequent sampling is done. However, frequent sampling is time intensive and expensive, often making it impractical². Additionally, grab sampling may under- or overestimate the actual pollutant concentrations outside of the sampling time^2,3,4. Consequently, loads calculated using such concentrations may not be accurate.

Alternatively, continuous monitoring provides accurate and timely information on water quality in a predetermined time interval, such as a minute, an hour, or a day. Users can select the appropriate time intervals based on their needs. Continuous monitoring enables the researchers, planners, and managers to optimize sample collection; develop and monitor time-integrated metrics, such as total maximum daily loads (TMDLs); evaluate the recreational use of the water body; assess baseline stream conditions; and spatially and temporally evaluate the variation of pollutants to determine cause-effect relationships and develop a management plan^5,6. Continuous monitoring of nutrients and sediment has recently received increased attention due to advances in computing and sensor technology, the improved capacity of storage devices, and the increasing data requirements needed to study more complex processes^1,5,7. In a global survey of over 700 water professionals, the use of multi-parameter sondes increased from 26% to 61% from 2002 to 2012 and is expected to reach 66% by 2022⁵. In the same survey, 72% of respondents indicated the need for the expansion of their monitoring network to meet their data needs⁵. The number of stations in a monitoring network and the number of variables monitored per station in 2012 are expected to increase by 53% and 64%, respectively, by 2022⁵.

However, continuous water quality and quantity monitoring in agricultural watersheds is challenging. Large rainfall events wash away sediment and macrophytes, contributing to high sediment load and debris buildup in the sensors and sondes. The runoff of excess nitrogen and phosphorus applied to agricultural fields creates ideal conditions for the growth of microscopic and macroscopic organisms and for the fouling of instream sensors and sondes, especially during the summer. Fouling and sediment buildup can cause sensors to fail, drift, and produce unreliable data. Despite these challenges, finer temporal resolution (as low as per minute) data are required to study the runoff processes and non-point source contamination, as they are affected by watershed characteristics (e.g., size, soil, slope, etc.) and the timing and intensity of rainfall⁷. Careful field observation, frequent calibration, and proper cleaning and maintenance can ensure good-quality data from the sensors and sondes, even at the finer time resolution.

Here, we discuss a method for the in situ continuous monitoring of two agricultural watersheds using multi-parameter water quality sondes, area-velocity and pressure transducer sensors, and autosamplers; their calibration and field maintenance; and data processing. The protocol demonstrates a way in which continuous water quality monitoring can be performed. The protocol is generally applicable to continuous water quality and quantity monitoring at any type or size of watershed.

The protocol was carried out in Northeast Arkansas in Little River Ditches Basin (HUC 080202040803, 53.4 km² area) and Lower St. Francis Basin (HUC 080202030801, 23.4 km² area). These two watersheds drain into tributaries of the Mississippi River. A need for monitoring tributaries of the Mississippi River was identified by the Lower Mississippi River Conservation Committee and the Gulf of Mexico Hypoxia Task Force to develop a watershed management plan and to record the progress of management activities^8,9. Moreover, these watersheds are characterized as focus watersheds by the United States Department of Agriculture-Natural Resources Conservation Service (USDA-NRCS), based on the potential for reducing nutrient and sediment pollution and for improving water quality¹⁰. Edge-of-field monitoring is being carried out in these watersheds as part of the statewide Mississippi River Basin Healthy Watershed Initiative (MRBI) network¹¹. More details of the watersheds (i.e., site locations, watershed characteristics, etc.) are provided in Aryal and Reba (2017)⁶. In short, the Little River Ditches Basin has predominantly silt loam soils, and cotton and soybean are the major crops, whereas the Lower St. Francis Basin has predominantly Sharkey clay soil, and rice and soybean are the major crops. At each watershed, in situ continuous water quantity and quality monitoring (i.e., discharge temperature, pH, DO, turbidity, conductivity, nitrate, and ammonium) was carried out at three stations in the mainstream using this protocol to understand the spatial and temporal variability in the pollutant loads and the hydrological processes. Additionally, weekly water samples were collected and analyzed for suspended sediment concentration.

Protocol

1. Site Selection

1. Watershed selection
   1. Select watershed(s) based on the magnitude of the pollution problem, priority of the watershed, proximity to the research facility, access to the site, and data objectives.
2. Stream sampling locations
   1. Select stream sampling location(s) based on the study purpose.
      NOTE: Optimal sampling locations are well-mixed within a cross-section, safely and easily accessible, geophysically stable (i.e., constant cross-section and a bank supportive of instrument station housing), and representative^12,13,14. Stations not immediately downstream from the confluence of two streams and in a straight channel section, without a converging or diverging channel cross-section, are more homogenous and representative¹⁴.
   2. Co-locate hydrological and water quality measurements at a cross-section to calculate the loads.
      NOTE: If identifying the spatial variation of nutrients and sediment in a watershed, select multiple stations to target potential sources throughout the watershed.

2. Instrument and Sensor Selection

1. Choose instruments and sensors to measure discharge and water quality and to collect water samples at the intended interval. Choose the instrument and sensors based on data need, watershed, and available resources.
   NOTE: Ideal sensors are reliable, accurate, sensitive, precise, low-cost, and suitable for the stream environment and require limited maintenance and minimal training of the field technician¹³. In an agricultural watershed, fouling and debris buildup are the greatest causes of concerns. Consequently, sondes equipped with self-cleaning and anti-fouling features are preferred.
   1. Use an autosampler, sondes, an area-velocity sensor, a pressure transducer, and a portable flowmeter.
      NOTE: The sonde should have a wiper to clean the turbidity sensor and a brush to clean the pH, ammonium, nitrate, and DO sensors.
      NOTE: The instrument in this protocol refers to a water sampling unit consisting of an autosampler, hose, strainer or flow module, and area-velocity sensor.
2. Select water quality parameters based on the data objective, sensor cost, and availability. Measure the temperature, pH, DO, conductivity, turbidity, ammonium, and nitrate every 15 min.
   NOTE: Temperature, pH, DO and conductivity are the most common parameters chosen and are measured at USGS stations, whereas nitrate, ammonium, and turbidity are less common but are gaining popularity^1,14.
   NOTE: The data objectives depend on the watershed characteristics. For example, nitrogen and phosphorus monitoring may be more important in agricultural watersheds compared to phosphorus monitoring in urban watersheds.

3. Sonde Calibration and Programming

1. Calibrate the sensors on the sonde as per manufacturer recommendations. Modify the calibration protocol as needed based on the local environmental conditions.
   NOTE: The frequency of calibration depends on the environment in which the sensors are exposed. Generally, it falls within 2 - 4 weeks. Here, the sondes are calibrated every 2 weeks during growing season and every 3 weeks in non-growing season (November to April).
2. At the laboratory, clean the sonde thoroughly before calibration. Clean the sensor surfaces using soft brushes (e.g., toothbrushes) and soap or all-purpose cleaner. Remove the circulator wiper and brush using a hexagonal Allen key; clean the wiper and brush.
3. Pour the electrolyte in the pH reference electrode, refill it with fresh electrolyte solution, and add a potassium chloride salt pellet to maintain the conductivity of the electrolyte solution. Close the cap so that it is airtight; some electrolyte will spill out while the cap is being screwed on. Rinse the sonde with deionized water.
4. Suspend the sonde on a sturdy support so that the bottom of the sonde rests approximately 20 - 30 cm above the table top, allowing for easy workability. Connect the sonde to the computer using a communication cable. Start the manufacturer's software. Press "operate sonde" to enter into the sonde program.
5. Set the number of calibration standards at the "parameter setup" tab.Calibrate the sensors in the following order: conductivity, pH, DO, turbidity, nitrate, and ammonium.
   NOTE: The order of the calibration is important, as nitrate and ammonium sensors use conductivity and pH values.
   NOTE: The number of calibration standards are 2 for conductivity, 2 or 3 for pH, 1 for DO, 2 or 4 for turbidity, 2 for nitrate, and 2 for ammonium.
6. Rinse the sensor(s) with DI water multiple times and dry the sensor(s) surface(s) with wipes before introducing a standard to the sensor to prevent cross-contamination.
   NOTE: Before calibrating each sensor, note the values the sensor reads for the following standards: DO, pH 7, turbidity for DI and 50 NTU, nitrate for 50 mg/L, and ammonium for 50 mg/L. These values can be used to evaluate whether the sensors were accurate in the field. They may also be prudently used to correct field values.
7. After the calibration of each sensor (steps 3.8 - 3.13) for a standard, "calibration successful" will appear; if the calibration fails, reset the sensor and try again. If the sensor still fails, the consumables may need replacement or the sensor may need factory repair.
   NOTE: Resetting the nitrate or ammonium sensor will reset both sensors.
8. Calibrate the conductivity sensor using 2-point calibration; 0 µs/cm for a dry sensor and 1,412 µs/cm for the standard solution. Choose "SpCond [µs/cm]" in the "calibration" tab.Dry the oval portion of the sensor completely with wipes. Enter "0.0" in µs/cm and enter "calibrate."
   1. Insert the standard in a pouch to entirely cover the oval portion of the sensor. Wait until the sensor reading stabilizes (~2 - 5 min), enter "1412" in µs/cm, and enter "calibrate." "Calibration successful" will appear; if the calibration fails, reset the sensor and try again.
9. Calibrate the pH sensor using pH 7 and pH 10 standards and check the linearity of the calibration with pH 4. Select the "pH[units]" tab in the calibration tab. Insert the pH 7 standard into a pouch covering both the pH junction and the reference electrode. Wait approximately 5 min for it to stabilize. Enter "7.0" as the pH value and enter "calibrate."
   1. Rinse the electrodes and dry them using wipes. Insert pH 10 and follow the same procedure as for pH 7. Insert pH 4 to check if the linearity of the calibration curve is met; the calibrated sensor should read 4 ± 0.2 for the pH 4.0 standard.
10. Calibrate the DO sensor using temperature-stabilized, air-saturated, deionized water (18 MΩ-cm) as single point standard.
    1. Select the "LDO%[Sat]" tab. Fill the calibration cup with DI water to the almost-full level and place the cup on the sonde. Invert the sonde to make sure that the temperature sensor and DO membranes are completely covered by the water.
    2. Wait approximately 5 min to stabilize the percent saturation reading. Once stabilized, enter "100" for the percent saturation. Enter the barometric pressure in mmHg by checking a local weather station and enter "calibrate."
       NOTE: DI water is temperature-stabilized and air-saturated by leaving it open to the atmosphere at least overnight in the laboratory for gas exchange, saturation, and temperature stabilization. Barometric pressure needs to be provided, since the DO saturation depends on atmospheric pressure in addition to the temperature (measured by the sonde itself).
    3. Check the scale factor, which should be 0.5 - 1.5, for acceptable calibration. Exit the calibration program, enter terminal mode, use the arrows to highlight "Log In," and press "enter." Highlight "level 3" and press "enter." Highlight "setup" and press "enter." Highlight "sensors" and press "enter." Highlight "DO" and press "enter." Highlight "DO% Sat" and press "enter." Note the scale factor.
    4. Press "Esc" to exit and enter "operate sonde" again. Select the "calibration tab" to continue the calibration.
    5. Invert the sonde back and suspend it so that the sensors face the ground.
11. Calibrate the turbidity sensor using 4 standards: DI, 50 NTU, 100 NTU, and 200 NTU.Select the "Turbidity[NTUs]" tab.In a calibration cup, put enough DI water to cover at least the bottom of the turbidity sensor. Let the turbidity reading stabilize. Enter point "1" for the DI standard, a "0.6" NTU turbidity value, and "calibrate."
    1. Similarly, calibrate the turbidity sensor for other standards. Prevent bubble formation by homogenizing the standards, turning the bottle up and down (do not shake) and pouring the standards along the cup.
    2. After calibrating all standards, check the sensor readings for DI and 50 NTU to see if the calibration was acceptable (i.e., within ±1%).
12. Calibrate the nitrate sensor using two standards: high (50 mg/L NO₃^--N) and low (5 mg/L NO₃^--N). Select the "NO₃^-[mg/L-N]" tab.
    1. Pour the 50 mg/L standard to fill the calibration cup up to three-quarters full and place the cup on the sonde, making a watertight connection. Invert the sonde so that the nitrate and temperature sensors are completely covered. Wait for 15 min (or until the reading is stable). Once stabilized, enter the standard level "1" and a value of "46.2." Record the temperature and mV readings in a notebook. Enter "calibrate."
       NOTE: The nitrate sensor uses the temperature sensor in addition to the conductivity and pH sensors.
    2. Rinse the sensors with DI water several times and dry them with wipes. Repeat the same procedure for the low standard. The difference between the two voltage readings should be 50 - 65 mV, and the difference between the temperature readings should not exceed 5 °F for the calibration to be acceptable.
13. Calibrate the ammonium sensor similarly to the nitrate sensor.
14. Reinstall and calibrate the wiper and brush. Choose the "SelfClean[Rev]" tab. Choose "1" rotation and enter "calibrate."
    NOTE: The wiper and brush will rotate one time.
15. Once all sensors are calibrated, program the sonde.Enter "set clock to pc time" in the "system" tab for synchronization.Delete the oldest log file if there are 4 existing log files and create a new log file.Once the log file is created, select the monitoring parameters and the parameters to log. Select the monitoring duration (i.e., until the next calibration, usually 2 - 3 weeks in agricultural watersheds) and interval (15 min) by choosing the start and end time of the log file and the logging interval. Save the log file.
    NOTE: At any time, a sonde can store up to 4 log files.
16. Check the internal battery voltage and replace the internal batteries if necessary.
    1. Select the "online monitoring" tab and start online monitoring.
    2. Check the internal battery voltage reading. If it is below 10.5 V, replace it with eight new C batteries.
       NOTE: The sonde stops recording data if the internal battery voltage drops below ~9.0 V.
    3. Use silicon sealant to seal the cap of the battery compartment to make a watertight connection.
17. Attach the sensor guard and put it in a bucket half-full of water.
    NOTE: The sondes in the bucket are ready for transport and (re)installation at the sites.The sondes must be submerged for the pH electrode to function properly.

4. Instrument and Sensor Installation

1. Area-velocity sensor and flow module
   1. Mount the area-velocity sensor securely on a steel plate at a selected cross-section. Mount the steel plate on the "L" bracket (Figure 1) that is mounted in the Telspar post driven at the thalweg of the stream (i.e., the deepest part of the channel) (Figure 1); the extension of the "L" bracket upstream of the Telspar post should be long enough so that the flow is not affected by the presence of the Telspar post in the stream. Place the sensor on the "L" bracket on the stream bed such that the tip of the sensor faces upstream along the flow lines.
      NOTE: The effect of Telspar post can be evaluated visually if the introduction of the post creates flow disturbance at the sensor position upstream or quantitatively using sensor readings with and without the Telspar post. In this protocol, cross-sectional variability was considered negligible. If it is to be evaluated, multiple sondes or sensors can be placed at a cross-section. The area-velocity sensor measures average velocity using the ultrasonic Doppler method. It does not require a conversion factor based on flow depth or velocity profiling and on-site calibration. The flow module measures velocity from -1.5 to 6.1 m/s and depth from 0.01 m to 9.15 m. As such, it is applicable to different watersheds.
   2. To calculate the discharge, measure the area of the cross-section.
      NOTE: The software can directly calculate the area if the shape of the channel or an equation is provided.
      NOTE: The data from the sensor are directly recorded in the flow module and can be downloaded to a computer using the manufacturer's software and a communication cable.

Figure 1. Layout of a Typical Instream Monitoring Station (Not to a Scale).
The station contains a Telspar post on which the sonde is suspended using a steel cable, a carabiner, and ferrules. The ferrules are not shown. The L-bracket on which the area-velocity sensor is mounted is placed at the stream bed and is secured tightly to the post using nuts and bolts. The autosampler (not shown in the figure) pulls the water sample from a hose that contains a strainer at the tip. The cable from the area-velocity sensor is connected to the flow module (not shown). Please click here to view a larger version of this figure.

2. Pressure transducer (PT sensor)
   1. Whenever the area-velocity sensor is unavailable, measure the depth using a pressure transducer.
   2. Install the PT sensor inside the Telspar post and secure it with a steel wire and ferrules; the tip of the sensor should just touch the stream bed. Program the PT sensor to measure the water depth at 15-min intervals.
3. Manual discharge measurement
   1. For stations with a PT sensor as a discharge measuring device, make a stage-discharge curve by manually measuring the discharge over a range of flows, covering at least low, medium, and high flows. Divide the cross-sectional area into several segments (30 - 60 cm wide), depending on the width of flow. Measure the mean velocity in the center line of the segment using a portable flowmeter. If the depth is <10 cm, measure the maximum velocity and multiply by 0.9 to get the mean velocity. If depth is 10 - 75 cm, measure velocity at 0.6 of the depth to determine the mean velocity¹⁵. For depths greater than 75 cm, measure velocities at three depths (0.2, 0.6, and 0.8 of the depth from the water surface) and average them¹⁵.
   2. Calculate the discharge of a segment using the average velocity, width, and depth of the segment and sum the discharges from all segments to obtain a total discharge.
   3. Follow the procedure for ranges of flows covering low, medium, and high flows.
   4. Determine the relationship between the stage (i.e., depth of flow measured by the pressure transducer at the time of the manual discharge measurement) and the measured discharges.
      NOTE: If the discharge is too high to measure the velocity manually, a temporary area-velocity sensor can be used make a relationship between the discharge measured by the area-velocity sensor and the depth measured by the PT sensor.
4. Water quality multi-parameter sonde
   1. Mount the sonde on the Telspar post with a steel wire, ferrules, and a carabiner for sonde safety and easy installation and removal (Figure 1). Place the sonde on the downstream side of the Telspar post to prevent damage from debris or wood logs that can come floating with the stream water, especially during flooding. Place the bottom of the sonde at least 1 - 10 cm above the stream bed to reduce the probability of sediment buildup on the sonde.
      NOTE: The sonde should always be submerged in the water. Therefore, in a stream with varying flows, the sonde should be high enough to reduce the buildup of the sediment on the sonde and low enough to prevent the sonde from getting exposed to air. However, for a channel with less variable flow, the sonde can be placed such that the sensors are approximately 10 cm below the water surface.
      NOTE: If the sonde has a depth sensor, the height of the depth sensor from the channel bed should be measured to account for the depth of installation of the depth sensor above the channel bed.
   2. Power the sonde with internal batteries and/or external batteries. Use a portable battery box to house the external battery and a communication cable to connect to the sonde. Program the sonde to collect data every 15 min and download the data directly to the computer using the communication cable.
5. Autosampler
   1. Install an autosampler in weather-protective housing at the top of stream bank on stable ground. Power the autosampler with a lead acid battery. Install a 20-W solar panel to charge the battery onsite.
   2. Secure a strainer pipe under water with the Telspar post or L-bracket and connect it to the autosampler with a hose.
      NOTE: The autosampler pulls water from the stream via the strainer and hose.
      NOTE: The positioning of the strainer pipe is important to obtain representative data. In this protocol, it was positioned assuming no cross-sectional variability.
   3. Program the autosampler to sample water weekly or based on need. Refer to the autosampler manual provided by the manufacturer.
      NOTE: The autosampler can be programmed to sample water based on rainfall, flow, time, or a combination. The sampler can be programmed to sample one sample into many bottles, many samples into one bottle (composite), or a combination.
      NOTE: The autosampler collects a volume of water (2,000 mL) necessary for the analysis of additional parameters in the laboratory. In addition to continuous water quality monitoring using the sonde, samples are analyzed on a weekly basis for suspended sediment concentration.

5. Sensor and Sonde Maintenance

1. Clean area-velocity sensor on every visit to reduce the debris on or near the sensor surfaces.
2. Frequently calibrate the sensors on the sonde.
   NOTE: Frequency is dependent upon season, hydrology, watershed, sensor type, and rate of fouling. In the watersheds chosen here, calibration was required every 2 weeks to collect good-quality data.
3. Replace the consumable parts as recommended by the manufacturer.
   NOTE: This includes a pH reference electrode/cap, a cap (membrane) for the DO sensor, ion-tip sensors (nitrate and ammonium sensors), and a circulating wiper and brushes.
4. Send the sonde for factory repair if necessary (i.e., if the sensor does not read acceptable values for the standards, even after resetting and recalibrating, or if the sensors fail calibration).

6. Field Sampling and Laboratory Analysis

1. Prepare in advance for the field trip to maintain the sensors and to collect the automatically collected water samples or manually sample and collect water samples if an autosampler is not available at the site. Make sure to include the items listed in the checklist (Table 1).
2. Collect the water samples in a clean (i.e., acid washed and rinsed) and dry jar (10 L), label them, and transport them on ice to the laboratory as soon as possible for analysis.
   NOTE: The collected water sample is a representative sample under actual conditions at the time of sampling and at the particular location; the integrity of the collected sample should be preserved against contamination and physical, chemical, and biological changes¹².
   NOTE: The container material required may be different for some analytes of interest, whereas acidification and/or filtration may be required at the site.
3. Analyze the collected water samples in the laboratory using standard methods before the approved holding times¹⁶.
   NOTE: Water samples can be analyzed using EPA 353.2; 4500-NO3 for nitrate, EPA 353.2; 4500-NO2 for nitrite, EPA 365.1; 4500-PI for phosphate, EPA 350.1; 4500-PJ for total nitrogen, EPA 365.4; 4500-PJ for total phosphorus, 2540-D for total suspended solids, 2540-C for total dissolved solids, and D 3977-97 for the suspended sediment concentration^16,17.
4. Follow the appropriate quality control and checks, such as blanks, standards, replicates, etc., during analysis. Follow the Quality Assurance Project Plan (QAPP).
5. Fill the chain of custody sheets for both the sample collector and the laboratory personnel and keep a copy of each. Note any unusual or notable events observed in the field on the chain of custody sheets.

7. Data Collection and Analysis

1. Collect water quality and quantity data from the sondes, flow module, and laboratory.
2. Save a copy of all raw data before working with the data correction and analysis.
3. Carefully inspect the collected data on turbidity and remove any zero (e.g., 0.0 NTU), NAN, or unreasonable values (e.g., 3,000 NTU; upper limit of detection of the sensor) before further analysis.
   NOTE: Caution should be exercised when removing any data. They are removed only when site-specific conditions in the field notes identify and determine that the data are not reasonable.
4. Use the stage-discharge relationship to calculate the discharge from the PT sensor.
   NOTE: The depth measured by the PT sensor must be pressure compensated.
   1. Use the manufacturer (In situ Inc.) software, "Baromerge," to post-correct the PT sensor data.
      NOTE: The data can be corrected by a fixed barometric pressure value by entering many barometric pressure values manually and automatically with a baroTroll log file. This protocol uses a baroTroll log file deployed at a nearby location to automatically correct the PT sensor data.
5. For area-velocity sensor data, remove any negative flow that could be sensor artifact.
   Caution: Sometimes there could actually be negative flow, depending on the site. In that case, do not ignore the negative velocity.
6. Calculate missing discharge data using a linear regression between upstream or downstream discharge and the discharge at the station.
   NOTE: The relationship should be statistically significant, which is usually the case between discharges for any upstream and downstream stations. In the watersheds tested here, the relationship was significant (p<0.01) and the correlation coefficient was greater than 93%. However, the missing discharge data can only be filled using this method if the distance between sites is short and the watershed characteristics remain similar.
7. Do not fill missing water quality data.
   NOTE: Water quality data are affected by many variables (i.e., timing and application of fertilizer, whether the discharge is increasing or decreasing, site specific conditions, etc.).
8. Perform a regression analysis between the suspended sediment concentration (SSC) from the laboratory results and the turbidity (NTU) measured at the stream.
   NOTE: Such a regression is sensitive to sediment size distribution, such that if sand constitutes a significant but variable fraction of the SSC, the regression will be poor. However, it can be improved if sands and fines are separated during sample analysis and if the fines are correlated to the SSC. Use the regression to calculate continuous SSC values.
9. Since pollutant concentrations vary with discharge, calculate flow-weighted concentrations using Equation 1⁶. Calculate the flow-weighted mean concentrations (FWMC) on a daily basis using hourly data. Alternatively, calculate it on an hourly basis using 15-min data; the FWMCs are time-integrated as well.
   
   where
   FWMC = flow-weighted mean concentration on a daily basis
   c_i = concentration of i^th sample
   t_i = time, 1 h
   q_i = discharge for i^th sample
   i = 1 to 24
10. Apply appropriate statistical techniques to meet the data objectives. When the data are non-normal, transform the data to make them normal or use the median ± interquartile range. Perform non-parametric tests for non-normal data.

Results

In the Aryal and Reba (2017) publication, this protocol was used to study the transport and transformation of nutrients and sediment in two small agricultural watersheds⁶. Additional outcomes from this protocol are described below.

Rainfall-runoff Water Quality Relationships:

The strength of continuous monitoring is that users can choose a fine tim...

Discussion

Overall, the continuous monitoring of nutrients and sediment has several advantages over monitoring using the grab sampling method. Hydrological and water quality processes are affected by rainfall over a very short span of time. Users can obtain high temporal-resolution data on nutrients and sediment to study complex problems. Other water quality parameters, such as conductivity, pH, temperature, and DO, can be obtained simultaneously and at the same cost as for monitoring nitrate, ammonium, and turbidity. Moreover, the...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Multiparameter sonde	Hach Hydrolab	DS5X	measures temperature, pH, conductivity, dissolved oxygen, nitrate, ammonium, turbidity
Area velocity flow module and sensor	Teledyne Isco	2150	measures average stream velocity and flow depth, and calculates flow rate and total flow based on provided cross-section area of the ditch. Stored data can be downloaded directly to computer.
Automatic portable water sampler	Teledyne Isco	ISCO 6712	automatically samples water in the set interval or in conjunction with flow module and sensor
Pressure Transducer	In-situ	Rugged Troll 100	measures presure, level and temperature in the water. Stored data can be directly downloaded to the computer
Portable flow meter	Flo-mate (Hach)	Marsh-McBirney 2000	For manual discharge measurement
Battery, 12 v, rechargeable	UPG	UB 1270	To power sonde
Battery, 12 v, rechargeable	Interstate Batteries	SRM 27	Lead acid battery to power autosampler
Solar panel	Alt E	ALT20-12P	To recharge battery at the site
C-8 batteries
Calibration standards	Hach or Fisher Scientific	mulitple	Standards of pH (4,7,10), conductivity (1412 uS/cm), nitrate (5 and 50 mg/L), ammonium (5 and 50 mg/L), and turbidity (50,100,200 NTU)
High nitrate standard	Hach	013810HY	50 mg/L
Low nitrate standard	Hach	013800HY	5 mg/L
High ammonium standard	Hach	002588HY	50 mg/L
Low ammonium standard	Hach	002587HY	5 mg/L
Turbidity standard	Fisher scientific	R8819050-500G	50 NTU
Turbidity standard	Fisher scientific	88-061-6	100 NTU
Turbidity standard	Fisher scientific	R8819200500 C	200 NTU
Potassium chloride salt pellets	Hach	005376HY	to maintain electrolyte for pH electrode
Potassium chloride standard	Fisher scientific	5890-16	1412 us/cm
Buffer solution, pH 4	Fisher scientific	SB99-1	for pH sensor calibration
Buffer solution, pH 7	Fisher scientific	SB108-1	for pH sensor calibration
Buffer solution, pH 10	Fisher scientific	SB116-1	for pH sensor calibration
Silicon sealant	Hach	00298HY	For sealing sensor battery cover water tight
All purpose cleaner	Sunshine Makers Inc	Simple green
Wipes	Kimberly-Clark
L-bracket
Telsbar post	Unistrut Service Company		Secure sensors and sondes in the stream
Steel wire			supend sonde and PT sensor
Carabiner			supend sonde and PT sensor
Allen wrench
Copper wire mesh	Bird B Gone		Rodent and bird control copper mesh roll
Adhesive Tape	Agri Drain Corporation		Tile tape, works in wet and cold weather