Operations Updates
ARM Climate Research Facility Operations Update - April 30, 2006
This bimonthly report provides a brief summary of significant accomplishments and activities in the operations area of the ARM Climate Research Facility (ACRF).
HydroKansas Follows Water Flowing Through Space and Time
Sets of rain and stream gauges like this one will provide information about water level and flow rates from 14 different sites throughout the Whitewater River watershed during the HydroKansas field campaign.
Beginning in May, the Whitewater River watershed in south-central Kansas is the setting for a 3-year field campaign hosted by the ACRF's Southern Great Plains (SGP) site. Called "HydroKansas," the goal of this research project is to develop a predictive understanding of floods on multiple spatial and temporal scales, using data collected from an area covering 1200 km2. In support of the field campaign, SGP technicians will maintain 28 rain gauges and 14 stream gauges within the watershed. These instruments expand on an existing observational network established within the watershed in 1997. Data from the expanded observational network will be transmitted in real time every 15 minutes into a relational database accessible from the Internet. Data collected during the campaign will be used to study the relationship between terrain, vegetation, water, and energy on daily, monthly, seasonal, annual and interannual time scales.
A watershed is a region of land where water drains downhill into a specified body of water, such as a river or wetland. It acts like a funnel, collecting all the water within the drainage area and channeling it into a waterway. The watershed area includes both the waterway and the land that drains to it. As an important factor in erosion processes and animal habitat, watersheds play a significant role in earth's hydrological and ecological systems. With instrumentation strategically distributed among 100 sites on streams of different sizes throughout the Whitewater River watershed, HydroKansas will focus on the primary variables relevant to studies of flooding and runoff generation. These variables include rainfall, stream discharge, infiltration, soil moisture, and evaporation and transpiration from hillslopes and riparian vegetation along the rivers. Evapotranspiration from riparian zones is an important component of land-atmosphere interaction, and HydroKansas will be one of the first experiments to directly account for moisture loss from riparian zones in the context of other hydrological and ecological measurements.
The stream gauging approach for HydroKansas involves an innovative method of using water level measurements from two submerged pressure transducers combined with a fluid mechanics-based model. The model uses information from a detailed survey of the streambed topography to estimate flow rates. To prepare for this campaign, SGP operations staff collected rain gauges from the observation network's remote sites for calibration prior to deployment. They also worked with area farmers on logistics for placement of the additional instruments so that deployment would occur as efficiently as possible. In October 2005, they began installing rain gauges at several of the network sites for data system and communications testing. SGP assisted with the streambed surveying process which began in January 2006, followed by final instrument calibration and deployment in the spring.
Disdrometer Joins Tipping Bucket to Improve Precipitation Measurements
At the SGP site, the disdrometer is installed near the site's main instrument cluster, approximately 50 feet east of the Central Facility. To avoid secondary splash contamination, the disdrometer's sensor cone is surrounded by splash-resistant material.
This spring, a pair of new distrometers began collecting data at the ACRF Southern Great Plains (SGP) site and the ACRF Darwin site in the Tropical Western Pacific (TWP), adding details about rain drop size distribution to the extensive collection of ARM measurements. A disdrometer is an instrument that measures the velocity and size distribution of falling hydrometeors (such as rain or hail). Accurate measurements of these properties are important for studying the evolution of water droplets in and near the melting layer—the region in the atmosphere where frozen precipitation melts and turns into liquid. Disdrometers can also be used to measure rain rate, which is currently obtained using "tipping buckets."
This tipping bucket, used to collect rain rate data at the TWP Darwin site, was joined in March by a disdrometer for obtaining measurements of drop size distribution.
Tipping bucket rain gauges are collocated with the disdrometers to verify their proper operation. Each rain gauge includes two buckets, each measuring 0.01 inches of water. When one bucket fills, it pivots to the side, bringing the second bucket into position. Each pivot closes the unit's magnetic switch and is counted by a data logger. This momentary switch closure, or pulse, can then be used to record rainfall rate and rain amount. While useful for rainfall rates, the tip bucket method cannot provide information on drop size distribution. With the disdrometer, drops that land on the sensor cone displace a very sensitive plunger. Momentum is transferred and the plunger displacement is directly related to the drop size.
In January 2005, the ARM Cloud Properties Working Group recommended the deployment of the disdrometers. The disdrometer at TWP began operating in January, followed shortly thereafter by the one at SGP in April. Measurements from the disdrometers will be used to test retrievals of size distribution obtained from the millimeter wavelength cloud radar, as well as to test an algorithm the radar uses to calculate rain rate. Rain drop distribution and rain rate data will be useful in the upcoming field campaign, the Cloud and Land Surface Interaction Campaign, or CLASIC, set to take place at SGP in 2007.
