Overview
Evaporation data can be calculated with a real time data system using an evaporation pan, a level sensor, and a rain gauge. Additionally, various weather parameters such as solar radiation, wind, and relative humidity can help you correlate the evaporation data to environmental events. Evaporation is calculated based on the amount of water that is lost throughout a day in an evaporation pan. Rainfall is used as a parameter to account for the amount of water that is added to the pan due to rain.
NexSens has simplified the sensor interface, data collection, telemetry and software associated with setting up and operating evaporation monitoring systems. Specifically designed for environmental monitoring applications, the iSIC data logger offers superior data acquisition performance in extreme conditions. Options for land-line, cellular, radio and satellite telemetry are available. The iSIC data logger is housed in a rugged, NEMA 4X enclosure with a long-life, rechargeable battery.
Data Logging and Telemetry
At the heart of the evaporation monitoring system is a battery operated data logger. It can be pole mounted, or housed in a secondary building or enclosure. Figure 1 shows a typical environmental evaporation measurement system.
When configured with telemetry, data can be transmitted from an evaporation site to a project computer.
Two factors help determine the best telemetry method site conditions and distance to a base computer. When the project computer can be located within a few miles (line-of-site) or few hundred feet (non-line-of-site), license free spread spectrum radio telemetry, such as the NexSens 4100-iSIC Data Logger and 4100-Base Radio is the best choice. The distance can be extended with use of repeaters, which for the NexSens system is simply an additional 4100-iSIC. When the project site is remotely located, relative to the project computer, cellular telemetry becomes the best option. The NexSens 3100-iSIC Cellular Data Logger is easily configured for various cellular providers and supports GSM/GPRS, CDMA, iDEN and EDGE technologies. Cellular signal strengths should be confirmed during the site survey and before setting up a cellular data account. When cellular signals are weak a NexSens 2100-iSIC Phone Data Logger or a NexSens 4200-iSIC Phone-Radio Data Logger should be considered.
Batteries and Solar Charging
All NexSens iSIC Data Loggers ship with as standard 12VDC, 8.5 AHr sealed lead acid battery. This battery is designed for typical applications and should work well in most evaporation monitoring applications. A NexSens A22, 20-watt solar panel will typically keep the battery above the threshold voltage required for proper operation. Consider an A23, 30-watt panel for extreme northern climates. Larger battery options accommodate non solar systems or projects which require other sensors and water samplers. The NexSens A03 12VDC, 26 AHr battery is designed to fit into the optional stainless steel enclosure. For ultra-high power applications consider NexSens A09 55 12VDC, 55 AHr deep cycle marine battery.
Class 'A' Pan
The U.S. Weather Bureau Class A Evaporation Pan is 1.2 m in diameter and 250 mm deep. These pans should be installed 150 mm off the ground. The normal operating water level is specified at 175 - 200 mm of water depth. This keeps the water level 50 - 75 mm below the rim of the pan. A level sensor located on the side of the Class A Pan is used to record the water depth. The Class A pan can be purchased with an automatic refill that will fill the pan back to the 200 mm depth as necessary.
Rain Gauge
A Rain gauge is required to monitor the amount of precipitation that falls in the evaporation pan. The measured rainfall is subtracted from the pan water level reading to determine the amount of water evaporated.
Level Sensor
A level sensor is required to monitor the amount of water currently in the evaporation pan. This data is used to keep track of how much evaporation has occurred. Additionally, when incorporated into a real-time system, it lets you know when to refill the pan.
Selecting a Level Sensor
There are three common types of level sensors used for evaporation monitoring
Pressure Sensors
A pressure sensor (Figure 2) measures the combined pressure exerted on it by the atmosphere and the head of water above it. A vented pressure sensor corrects for changes in barometric pressure. The sensor is fixed below the minimum expected water level and a cable containing the sensor signals and vent tube runs from the sensor to the data logger.
Bubblers
Bubbler systems (Figure 3) measure the water level by detecting the pressure required to force an air bubble through a submerged tube. The pressure is proportional to the water level. A bubble tube runs from the bubble generator, which is typically mounted near the data logger to a surface water orifice. The orifice is fixed below the minimum expected water level and diffuses the air bubble into the water. Bubbler systems offer high accuracy, which can be slightly degraded by sediment build-up.
Shaft Encoders
Shaft encoders (Figure 4) connect to floats which follow changes in water level. As the float position changes, the shaft turns and the encoder tracks the position. The least expensive of all evaporation monitoring level sensors, shaft encoders offer an affordable alternative and when properly installed high accuracy and reliability. The installation will require a stilling well, which for some sensors like the OTT Thalimedes can be a simple 4-inch pipe with locking cap.
Evaporation Pan Location
The evaporation pan location is one of the most important considerations for obtaining reliable evaporation data. The pan should be placed near the site of interest but not on bare ground or next to gravel areas or black top. These areas increase evaporation because of abnormal temperatures relative to the rest of the landscape. The pan should be kept away from trees or near buildings where it will be shaded part of the day. The vegetation within a 10 m radius of the pan should be kept mowed. An ideal location is a grassed area next to or in the site to be monitored. The conditions should be representative of what the area is experiencing. Take into account weather, amount of sunlight and typical temperatures. The pan should be raised 150 mm off the ground, leveled and firmly supported.
Software
NexSens iChart software (Figure 5) is a Windows based program, which simplifies and automates many of the tasks associated with acquiring, processing, analyzing and publishing evaporation data. iChart has an extensive library of predefined device drivers for popular water level sensors. New monitoring devices are quickly configured by selecting from a drop-down list of manufacturers. There is no complicated programming required to set up data loggers and real-time monitoring networks. Simply choose a sensor, select a telemetry option, and your computer is ready to begin retrieving data. Data processing, reporting, exporting and sharing is automated and easy. One button export to Microsoft Excel and Adobe PDF are notable features.
Web Datacenter and Web Data Applets
WQData (Figure 6) is a secure web datacenter providing an online interface for viewing evaporation data. It offers 24/7 instant access to project data using any web browser. Specifically designed for environmental monitoring applications, WQData is easy to setup and maintain. Your project specific web datacenter is automatically generated by iChart software and posted to the NexSens servers. The datacenter can be setup as a secure (password protected) site or an open site for easy access by anyone with the web address.
Included with WQData is the NexSens Web-Data Applet. Simply embed the applet code on your webpage to present website visitors with project data. An 'ALL DATA' button on the applet directs the visitor to the project datacenter where project maps, data, tables, graphs, statistics and other data visualization tools provide a professional and fully featured project website. An integrated project forum allows all team members to communicate and share information.
