Airborne Gamma Survey Program

Overview
Operational Snow Survey Program
Operational Flight Line Network: Survey Locations
Airborne SWE Measurement Theory
Sensor/Instrument Description
Airborne Snow Water Equivalent Measurement Practice
Ground-based Snow Water Equivalent Measurements
Airborne Snow Survey Program Products
Explanation of Airborne Gamma Data Format (SHEF)
Winter Airborne Snow Survey Schedule
Summary

Overview

The National Operational Hydrologic Remote Sensing Center (NOHRSC) has developed, and currently maintains, an operational Airborne Gamma Radiation Snow Survey Program to make airborne Snow Water Equivalent (SWE) and soil moisture measurements. Airborne SWE measurements are used by NWS Weather Forecast Offices (WFO) and NWS River Forecast Centers (RFC) when issuing river and flood forecasts, water supply forecasts, and spring flood outlooks.

Both SWE and soil moisture tend to be spatially variable across the landscape, making it difficult to collect an adequate number of representative ground-based point measurements. Using airborne gamma radiation detection methods, information on SWE and soil moisture is collected over entire flight lines, providing a more representative sample of the mean areal conditions on the ground. Airborne gamma radiation measurement remains one of the most successful methods of acquiring spatial information on SWE and soil moisture. NOHRSC uses this information to generate maps of the spatial distribution of SWE for use by NWS River Forecast Centers to assist in streamflow and flood forecasting. Two NOAA aircraft are operated full-time by NOHRSC thoughout each winter and spring to collect this information from across the United States. Data from each survey are transmitted directly to NOHRSC, where the data are archived, processed, and made available to NWS forecast offices, other government agencies, and to the general public.

This document describes key aspects of the Airborne Snow Survey Program and the technique used to make airborne snow and soil moisture measurements using natural terrestrial gamma radiation. It provides a brief description of the following:

• How airborne gamma radiation SWE measurements are made;

• What the airborne measurements represent and how (and why) they differ from ground-based SWE measurements;
  

• How the measurements can be modified by Service Hydrologists and other end-users using alternative soil moisture assumptions;
  

• How and when airborne data are transmitted to NWS field offices; and
  

• Details of snow survey mission scheduling and RFC communication necessary to ensure an effective airborne data collection season.

Operational Snow Survey Program

NOHRSC maintains the Operational Airborne Gamma Radiation Snow Survey Program from its office near Minneapolis, MN. The program makes airborne SWE and soil moisture estimates over a network of 1900 flight lines covering portions of 25 states and seven Canadian provinces.

The NWS owns a twin-engine Aero Commander dedicated to provide program support 12 months of the year. Two additional NOAA aircraft have been calibrated for airborne radiation data collection and are available for program support: a second Aero Commander and a Turbo Commander. NOAA Corps Commissioned Officers serve as program pilots and live in the Minneapolis area. The NOHRSC maintains two airborne gamma radiation detection packages for use in any two of the three calibrated NOAA aircraft. Additional NOAA Corps pilots are available during the intensive snow survey season.

Airborne SWE data are collected simultaneously by two snow survey aircraft almost continuously from late January through mid-April each year. The data are transmitted electronically from the field to Minneapolis where they are checked for accuracy, entered into Standard Hydrometeorological Exchange Format (SHEF), sent to the North Central RFC (NCRFC) Gateway computer, and transmitted to NWS field offices within 2 to 3 hours after either aircraft lands for fuel at noon or in the evening anywhere in the country.

Over 1,000 real-time airborne SWE measurements are made by both aircraft during a typical snow survey season. In addition, the pilots routinely take VHS video imagery over the flight line network during each snow survey mission. The airborne computer calculates the SWE immediately after a flight line is flown and a voice synthesizer connected to the aircraft intercom transfers the survey results to the audio channel of the video tape.

The video tapes, with the SWE data encoded on the audio track, can be sent via overnight mail to the appropriate RFCs or WFOs where they can be viewed by the hydrologists. Frequently, the video imagery gives the RFC hydrologists some feel for the current snow cover conditions, river ice conditions (whether a river is frozen, partially open, or fully open with flowing water), and ground ice conditions. Upon request, 35mm prints or slides are taken of selected areas during a survey.

Typical airborne snow survey operations span January through April each year using two aircraft simultaneously. Each flight line is typically 16 km long and 300 m wide, covering an area of approximately 5 km². Consequently, each airborne snow water equivalent measurement is a mean areal measure integrated over the 5 km² area of the flight line.

Operational Flight Line Network: Survey Locations

The Airborne Program provides real-time SWE and soil moisture data to NWS offices in the Eastern, Central, and Western Regions for a pre-determined network of over 1900 flight lines. These flight lines are located in mountainous and northern areas of the Nation that have significant annual snow accumulations.

The map below shows the locations of each of the flight lines maintained by NOHRSC. Click on any of the grid boxes on the map to see a detailed view of the flight line locations.

To see the map for a particular area, click on the index map.
Maps contain a labelled GIF of flight lines within a 4° x 4° block.

An ascii text description of the NOHRSC flight line database includes the name, river basin, latitude, longitude, elevation, and length.

Airborne SWE Measurement Theory

The ability to make reliable airborne gamma radiation SWE measurements is based on the fact that natural terrestrial gamma radiation is emitted from the potassium, uranium, and thorium radioisotopes in the upper 8 inches of soil. The radiation is sensed from a low-flying aircraft flying 500 feet above the ground. Water mass in the snow cover attenuates, or blocks, the terrestrial radiation signal. Consequently, the difference between airborne radiation measurements made over bare ground and snow-covered ground can be used to calculate a mean areal SWE value with a root mean square error of less than one-half inch. The technique measures the attenuation of the radiation signal due solely to the intervening water mass.

The technique provides no information on snow depth, only SWE. Airborne SWE is calculated using the following relationship:

Where:

C and C_o = Uncollided terrestrial gamma count rates cover snow and bare ground, respectively

M and M_o = Percent soil moisture over snow and bare ground, respectively

A = Radiation attenuation coefficient in water (cm² g^-1)

The inverse radiation attenuation coefficients used in Eq. (1) for the potassium, thorium, and total count windows are 14.34, 18.85, and 17.73, respectively. An independent SWE value is calculated for each of the three radioisotope photopeaks. A weighted SWE is calculated by multiplying each of the three independent SWE values by a weighting coefficient (which sums to unity) and summing the results. The potassium, thorium, and total count weighting factors are 0.346, 0.518, 0.136, respectively. Only the weighted SWE value is reported; the three SWEs calculated for each photopeak are available but not typically reported.

Background radiation and soil moisture values (C_o and M_o) are collected once under no-snow cover conditions and used to calibrate flight lines. It is not necessary to collect background calibration data more than once for a specific flight line.

A normalized calibration count rate is derived using the background radiation and soil moisture data. Because the strength of the radiation signal is a function of only soil moisture in the upper 8 inches, the normalized calibration does not change over time. In addition to the normalized calibration value, we also need an over-snow radiation measurement (C) and a mean areal estimate of the percent soil moisture (M) in the upper 8 inches over the flight lines before we can make an airborne SWE calculation. An estimate of the soil moisture is required for the upper 8 inches because the radioisotope source of radiation sensed by the airborne detection system is in the upper 8 inches of soil. Terrestrial radiation concentrated below the upper 8 inches of soil is blocked, or attenuated, by the upper 8 inches of soil.

Flight lines are typically 10 miles long and 1,000 feet wide covering an area of 2 to 3 square miles. Radiation data collected over each flight line are an integrated value for the 2 to 3 square mile area. Consequently, airborne SWE measurements are a MEAN AREAL measurement made over the 2 to 3 square mile area of a flight line. The airborne measurement technique integrates the variability of the snow cover over the flight line and reports one SWE value which represents the AVERAGE water equivalent for the flight line. Mean areal SWE measurements tend to more accurately reflect the local snow cover conditions than do single-point observations taken in a snow drift or in a blown-clean area.

Percent Soil Moisture by Weight.

Percent soil moisture by weight is calculated as the weight of water divided by the weight of dry soil multiplied by 100. By this formulation, it is possible to have a percent soil moisture value greater than 100 percent. Field holding capacity is largely a function of land use and soil type. For a typical loam soil, field holding capacity is about 35 percent soil moisture. Soil moisture can generally be characterized by the relationship given in table 1.

Under frozen soil conditions, it is possible to accumulate interstitial ice which can raise the percent soil moisture value for the upper 8 inches to typical values of 50 to 70 percent.

For more detailed information about the phsyics of the airborne gamma SWE measurement, see:

Sensor/Instrument Description

The airborne detector package consists of five downward-looking 10.2 x 10.2 x 40.6 cm NaI(Tl) scintillation detectors; two 10.2 x 10.2 x 20.3 cm, upward-looking detectors (used to isolate the effects of the random gas contribution); a pulse height analyzer (PHA); a Hewlett-Packard 9825 minicomputer used to reduce and record the output data onto magnetic tape; temperature, pressure, and radar altitude sensors; and a remote control unit used by the system operator or navigator to control and monitor the data collection.

Airborne Snow Water Equivalent Measurement Practice

The airborne measurement technique depends on: (1) the difference between the no-snow and over-snow radiation measurements (Co and C) and (2) the effect of the soil moisture conditions (M) extant at the time of airborne SWE measurements. The technique is not sensitive to the phase of the moisture in the snow or soil and accounts for the effect of atmospheric moisture. The NOHRSC does not, however, collect radiation data when it is raining or snowing and requires visual flight regulation conditions to collect airborne data at 500 feet above ground level. Agricultural vegetation does not significantly affect the no-snow or over-snow radiation measurements.

Figure 2 is a schematic representation of the various factors associated with making and understanding airborne SWE measurements. It shows that the airborne SWE value totals the moisture (regardless of phase) contained in the snow water equivalent (SWE' ), ice lenses (IL), liquid water in the snowpack (LW), ground ice (GI), standing water (SW), and superimposed soil moisture (Ms). It also shows that the total soil moisture (Mt) in the 8-inch soil zone is composed of the primary soil moisture (M) and the superimposed soil moisture (Ms). The above characterization makes it possible for an end-user to "recalculate" the reported airborne SWE value by changing the primary soil moisture (M) to a more representative and alternative primary soil moisture value (M' ).

To better understand what the airborne SWE measurement represents, it may be useful to consider three different situations posed as a series of questions:

• How much total liquid and solid moisture has accumulated since the late fall? In this case, M should be set to a value which represents the fall soil moisture condition. Ms will, quite likely, be significant and represent moisture from later precipitation, midwinter snowmelt, and/or moisture transported from the lower soil zones toward the surface due to winter vapor pressure gradients near the soil surface. Consequently, the resulting airborne SWE value will include a significant amount of moisture in the soil (Ms) which exceeds the fall soil moisture value (M).

• How much above-ground moisture (SWE' + IL + LW + GI + SW) and soil moisture (Ms) above field-holding capacity is present? In this case, the primary soil moisture value (M) is set to field-holding capacity of the local soil (typically 35 percent). This is an appropriate question in many cases because the answer includes that portion of the soil moisture above field-holding capacity which acts hydrologically very much like snow water equivalent.

• What is the total above-ground moisture (i.e., the traditional SWE value)? The answer is derived by setting the superimposed soil moisture value (Ms) to zero and letting the primary soil moisture (M) equal the total soil moisture (Mt). In this case, the value of M can easily be above field-holding capacity and values of 50 to 70 percent soil moisture are not uncommon. If M is an accurate estimate of the total mean areal soil moisture in the upper 8 inches over a flight line, then (and only then) the resulting airborne SWE will be an accurate estimate of the above-ground moisture content. In this case, however, the airborne SWE value will NOT account for the 15 to 30 percent soil moisture (1 to 2 inches of water equivalent) in the soil above field-holding capacity which typically exists and acts hydrologically like snow water equivalent.

In summary, airborne SWE measurements made using a value of 35 percent or less soil moisture for (M) tend to generate a higher estimate than ground-based SWE measurements, which tend to underestimate true SWE conditions on the ground. Consequently, airborne snow measurements should be expected to be a better measure of true ground snow cover conditions than those provided by NWS cooperative observers and others using traditional ground-based snow measurement techniques. It is essential to understand this concept in order to fully understand the nature of airborne snow measurements.

Ground-based Snow Water Equivalent Measurements

Ground-based SWE measurements are typically made by an observer using a snow tube and a scale, which give a direct reading of SWE. Snow tube measurements tend to systematically underestimate true SWE because of the sampling difficulties associated with ice lenses, ground ice, and depth hoar. It is virtually impossible to accurately measure the water content in ice layers on the ground that can be 2 to 4 inches thick. (An alternative procedure used to make SWE measurements is to melt the snow sample to derive the water equivalent, a technique encumbered with equal difficulty.) Additionally, one point sample tends not to be representative of an area.

Airborne SWE values, however, include all the SWE, liquid water, ground ice, and standing water above the ground. In addition, they may also include some superimposed soil moisture (Ms) depending on the value of the primary soil moisture (M) used in the SWE calculation. As a result of the tendency for ground observations to underestimate true SWE and the possibility that the airborne SWE value may include some soil moisture (depending on the value of M used), ground-based SWE values tend to underestimate airborne SWE measurements. Additionally, the reliability of one point estimate to characterize the mean areal SWE of a 2 to 3 square mile area is suspect. (The NOHRSC typically makes over 1,000 snow depth and density measurements on the ground over a single flight line when we want a good ground measurement of mean areal SWE to compare with airborne measurements.)

Airborne Snow Survey Program Products

All Airborne Snow Survey Program products are sent out over NWS communications using the SHEF ID of MSPRRMASP and to the NOHRSC's web site in near real-time. The SHEF IDs should be in the database of appropriate NWS offices in order to receive the airborne data. The operational airborne data are transferred to the NOHRSC from the aircraft each noon and evening. The data are checked for accuracy, entered into SHEF, and sent to the NCRFC Gateway computer. The product is then sent from the RFC Gateway computer to NWS field offices approximately 2 to 3 hours after the survey aircraft land each day anywhere in the country.

A typical airborne survey might take 3 or 4 days for one specific region. As many as two or three airborne snow data products could be issued each day during peak snow survey season. Airborne SWE products are provided in graphical (figure 3) and text formats. Service Hydrologists should ensure that the products appropriate for their area are archived in their database.

Text products provide airborne SWE data in SHEF and include the flight line number, date of observation, percent snow cover (%SC), airborne SWE in inches, airborne SWE calculated using a value of M equal to 35 percent soil moisture (SWE 35%), percent soil moisture (M) used to calculate the SWE (%SM(M)), the type of estimate for %SM(M), the date of a fall soil moisture survey (if conducted), the fall percent soil moisture (%SM(F)), and pilot remarks when appropriate.

Explanation of Airborne Gamma Data Format (SHEF)

The data from each survey is formatted into a specific NWS format called a SHEF message. These messages are available from the Operational Products section of the NOHRSC web page, as is a guide to interpreting the airborne gamma survey SHEF messages.

Winter Airborne Snow Survey Schedule

There is no winter airborne snow survey schedule that is set and strictly adhered to well in advance of the accumulation season. The locations and dates for airborne snow surveys are established in real-time and are based on national snow cover conditions. Some airborne snow surveys are scheduled around the issuance dates for spring flood outlook products. The RFCs typically issue four flood outlooks. The outlooks are scheduled for release to the public by the WFOs on the first and third (or second and fourth) Fridays of February and March. Airborne snow data are typically collected over critical areas of the country before all but the first flood outlook. Each RFC designates a focal point to interact with the NOHRSC on survey scheduling and related matters.

Airborne survey schedules and basins for data collection are determined approximately 10 days before each outlook release date. Consequently, it is important for the designated Airborne Program focal point in each RFC to contact the NOHRSC office to discuss the need for airborne surveys and video coverage in their area before each flood outlook, if warranted by existing snow cover conditions. Service Hydrologists should have similar interactions with the NOHRSC to coordinate additional requirements if they exist.

Summary

Reliable airborne SWE and soil moisture measurements are provided in real-time to NWS field offices upon request from NWS hydrologists. The airborne data are collected routinely over a network of more than 1900 flight lines in 25 states and 7 Canadian provinces from January through mid-April. Airborne snow survey missions and basins for coverage are based on input from NWS field hydrologists and snow cover conditions across the country. The airborne SWE data are used to assess:

1. The above-ground moisture held in the snowpack in the form of snow, ice lenses, ground ice, and standing water;
2. The above-ground moisture plus some measure of the soil moisture above field-holding capacity (which tends to respond hydrologically in a fashion similar to that of SWE); and/or
3. The aggregate change in both the above-ground moisture and the total soil moisture from some previously established measurement (e.g., a fall airborne soil-moisture survey).

The airborne snow data collected in the late winter and early spring are used by hydrologists from the NWS and other Federal and state agencies when issuing spring flood outlooks, water supply outlooks, and river and flood forecasts. Additionally, airborne soil moisture data are used operationally each fall over large areas of the Upper Midwest to assess the antecedent soil moisture conditions immediately before winter freeze-up.

The airborne data are transferred electronically from the snow survey aircraft to the office in Minneapolis. They are checked for accuracy, entered into SHEF, and sent to NWS field offices within 2 or 3 hours after the aircraft lands each noon and evening for fuel.

The Service Hydrologist has the ability, with the information provided on the web site, to "reprocess" any airborne SWE value reported using a "new" and/or more representative primary soil moisture value (M) in the original airborne SWE calculation.

Listed on the web site are several recent (and several not-so-recent) publications which describe the airborne measurement technique, research results, and other aspects of the Airborne Snow Survey Program of the NOHRSC. Copies of the publications are available upon request.