Alaska; Optimal Storage Volumes For Rainwater Catchment Systems In Alaska

Optimal Storage Volumes for
Rainwater Catchment Systems in
Alaska

By

Greta Myerchin, Amy Tidwell, Bill Schnabel and
Daniel White

March 2008

ATTAC Research Report # 200801

TABLE OF CONTENTS

INTRODUCTION............................................................................................................. 1
Rainwater Catchment System and Storage Requirement ............................................... 1
Objective ......................................................................................................................... 1

STORAGE RESERVOIR DESIGN................................................................................ 2
Storage Reservoir Capacity............................................................................................. 2
Water Usage.................................................................................................................... 2
Roof Collection Area ...................................................................................................... 3
Climate and Threshold Temperature .............................................................................. 3
Precipitation Design Year ............................................................................................... 3

PROCEDURE ................................................................................................................... 4
Community Selection...................................................................................................... 4
Optimization Protocol..................................................................................................... 4

RESULTS .......................................................................................................................... 5

SUMMARY ....................................................................................................................... 6

REFERENCES.................................................................................................................. 7

APPENDIX A: Design Year Climate Data for Selected Communities............................ 8

APPENDIX B: Calculation of Current Storage Condition............................................... 9

APPENDIX C: Graphs of Optimal Rainwater Storage Volume for Twelve Alaskan
Locations........................................................................................................................... 10

Optimal Storage Volumes for Rainwater Catchment Systems in Alaska i
ATTAC: Alaska Training/Technical Assistance Center

INTRODUCTION

Rainwater Catchment System and Storage Requirement
Rainwater catchment systems are designed to capture and store rainwater for use as a
primary or supplemental water source. In residential homes, stored rainwater can be used
for landscape irrigation or household services such as toilets and laundry. Collected
rainwater may also be used as a source of potable water, although it is recommended that
the stored water be properly tested and/or treated prior to this use.

Design of rainwater catchment systems requires careful consideration of storage capacity.
Inappropriate sizing of a storage tank or reservoir can negatively affect a homeowner in
many ways. Overestimation of the storage requirement can result in an oversized and
needlessly expensive storage system. Undersized storage tanks, on the other hand, may
not fulfill the homeowner’s water use needs.

Objective
The objective of this report is to present computer modeling results intended to estimate
optimal rainwater catchment system storage requirements. The resultant optimal storage
volumes were calculated as a function of water demand, catchment area and local
climatic conditions for twelve Alaskan localities. The data are intended to serve as
storage basin design guidelines to be used during system planning. The guidelines are
intended to supplement, but not replace, the professional judgment of experienced system
designers with site-specific knowledge.

This reports describes the optimal storage capacity for systems designed to collect and
store rainwater during the operational season only (i.e., during non-frozen conditions).
Optimal storage volume is defined here as the minimum tank volume necessary to meet
water use demand through the operational season while maintaining a reserve capacity of
at least 5%.

Optimal Storage Volumes for Rainwater Catchment Systems in Alaska 1

STORAGE RESERVOIR DESIGN
A variety of factors influence storage requirements. In this study, factors relevant to
Alaskan rainwater catchment systems were considered. Such factors included catchment
surface area, collection efficiency, water use rates, and climatic characteristics.

Storage Reservoir Capacity
This study evaluated storage reservoirs with capacities ranging from 100 to 20,000
gallons. Storage tanks fabricated to hold volumes of 100 gallons or less are often used
for transport or as break tanks, but are not often used as primary rainwater storage tanks.
In regions of Alaska where freezing conditions persist, residential exterior tank sizes do
not commonly exceed 2,500 gallons, based on physical and insulation restrictions as well
as product availability (Greer, 2007). In circumstances requiring storage of large
volumes (i.e., greater than 2,500 gallons), interior or underground cisterns may be
utilized. Alternatively, captured rainwater can be routed to natural ponds or engineered
impound structures for storage.

Water Usage
An estimate of water demand was required in order to estimate the appropriate rainwater
reservoir volume. In practice, water demand varies from person to person and according
to lifestyle considerations. While water usage in most Alaskan households is similar to
average US household water use rates, a relatively large proportion of Alaskans reside in
limited water use households. Examples of limited use households include those without
flush toilets, clothes washers, or shower facilities. An estimate of twenty gallons per
capita per day (gpcd) is commonly used when households contain no flush toilet. If a
household contains a flush toilet, water use can increase to 70 gpcd (Johanson, 2002).
Other authors estimate the household water use for a typical family of four to range from
150 to 200 gpd (Siefert, 2004). This report presents results based upon assumed
household water use rates ranging from 5 gpd to 250 gpd.


Roof Collection Area
Rainwater catchment area was an additional model input parameter. In order to
effectively interpret the model results, a system designer must estimate the catchment
surface area of the system being designed. If the entire home roof surface is used for
rainwater collection, this can be accomplished by calculating the horizontal surface area
under the roof, including the eaves. Reducing the estimated area by 10% will account for
loss by flushing, overflow, and other loss factors (Johanson, 2002).

Climate and Threshold Temperature
In this study, it was assumed that liquid precipitation was available for rainwater
catchment, but frozen precipitation (i.e., snow, sleet) was only available as spring
snowmelt. Additionally, it was assumed that the total volume of accumulated snow in the
catchment system was sufficient enough to fully recharge the storage reservoir upon
melting. Consequently, each modeled operational season began in the spring with a full
reservoir.

The modeled operational seasons in this report were bound by threshold periods, defined
as 15-day periods during which the average daily air temperature was above or below
specified threshold temperatures. As a result, the operational periods were considered to
begin on Day 1 of the thaw threshold period, and end on Day 15 of the freeze-up
threshold period. In this study, the mean threshold temperature used for calculating thaw
was 0.1 oC, while the mean threshold temperature used for calculating freeze-up was -1.9
o
C, based upon values recommended by Hinzman (1990).

Precipitation Design Year
The model output presented in this study was based upon typical precipitation patterns
experienced in specific Alaskan localities, as represented by the site-specific design year.
The design years represented actual climate data (e.g., precipitation patterns and thaw
season length) considered to most closely match average conditions for each site studied.
A table located in Appendix A provides further information with respect to the design
year characteristics of each modeled location.


PROCEDURE

Community Selection
Selection of the modeled communities was based upon population, climatic
characteristics, and location. Twelve relatively large regional hubs were selected in order
to represent a range of climatic characteristics experienced in various regions throughout
Alaska. The modeled communities included Anchorage, Barrow, Bethel, Delta Junction,
Dutch Harbor, Fairbanks, Juneau, Ketchikan, Kotzebue, Nome, Tok, and Valdez.
Descriptions of the climatic characteristics for each listed community are presented in
Appendix A.

Optimization Protocol
An automated evaluation process was devised to estimate the optimal storage volume
required for each community. The procedure was written in Visual Basic for
Applications and was implemented within Microsoft Excel software. The program first
determined a representative year for design purposes. This design year was selected from
a record of site-specific climate data, and represented the year most indicative of average
site conditions. Next, the program calculated individual tank profiles for a combination
of collection areas, usage rates, and storage capacities. The resultant tank profiles were
graphic representations of the daily water balance, as summarized in the following
equation:

Current Storage = Initial Storage + Precipitation Inflow – Water Consumed

Please see Appendix B for a more detailed description of the water balance calculations.

Storage optimization graphs, located in Appendix C, were prepared for each community
based upon the results of the water balance algorithm. From these graphs, users can
determine the estimated optimal rainwater storage volume under a given set of
conditions. As described previously, optimal storage capacity is defined as the volume of
storage necessary to meet the water use requirements at all times during the operational
season while maintaining a 5% reserve volume.


RESULTS
As described above, optimal storage reservoir volumes were estimated for twelve
Alaskan communities. The resultant estimated optimal volumes are presented in the
graphs in Appendix C. In order to obtain the optimal storage volume for a system under
consideration, the reader is asked to perform the following steps:

1) Identify the listed community with a climate most similar to that at the location of
the system being designed. Use the table presented in Appendix A and/or other
climate resources for this step. Use this community’s graph (Appendix C) for the
remainder of this procedure.
2) Estimate the anticipated water demand (gpd) and catchment surface area (ft2) for
the system under consideration, using recommendations provided in this report or
elsewhere.
3) On the appropriate community-specific graph, select the surface area curve
corresponding to the catchment surface area of the system under design. Identify
the point on that curve corresponding to the anticipated water demand listed on
the vertical axis.
4) The optimal reservoir volume is represented by the point on the horizontal axis
underlying the point at which the selected surface area curve meets the level of
anticipated water demand.


SUMMARY
Two-thirds of Alaska’s population is serviced by a public water supply, while the
remaining third utilize independent sources such as private wells or commercial delivery
(USGS, 2004). Rainwater collection is an alternative independent water supply practice
that can not only benefit households with little or no available water, but can also reduce
the demand on municipally-managed water resources. In order to aid in the design of
rainwater catchment systems statewide, this study was conducted to estimate optimal
rainwater storage requirements for a group of Alaskan communities experiencing a range
of climate conditions. Optimal rainwater storage volumes for twelve Alaskan locations
can be determined from the graphs in Appendix C.


REFERENCES

1. Greer Tank and Welding, Inc.; http://www.greertank.com/PolyTanks.htm (2007)
2. Hinzman, L.D.; The Interdependence of the Thermal and Hydrologic Processes of
an Arctic Watershed and their Response to Climate Change; (1990) University of
Alaska, Fairbanks.
3. Siefert, R. D.; Suggestions for Installing Domestic Water Storage Tanks; (2004)
University of Alaska, Fairbanks Cooperative Extension Service; Building in
Alaska Series; #HCM-04950.
4. Alaska Division of Community Advocacy (ADCA) website:
http://www.commerce.state.ak.us/dca/commdb/CF_COMDB.htm. Demographic
information for Ketchikan and Fairbanks.
5. Translators: Johanson N., Seifert R.D. Water Cistern Construction for Small
House; (2004) University of Alaska, Fairbanks Cooperative Extension Service;
Alaska Building Research Series; #HCM-01557. From a Norwegian publication:
BYGGFORSK, NBI A515.161


Appendix A

Design Year Climate Data for Selected Communities

Operational
Operational Operatioal Season End
Design Available Season Length Season Begin Date
Community Year Rainfall (inches) (days) Date (Thaw) (Freezeup)
Anchorage 1977 11.3 212 26-Mar 24-Oct
Barrow 1986 2.6 107 5-Jun 20-Sep
Bethel 1978 11.0 171 11-Apr 29-Sep
Delta Junction 1960 8.8 172 14-Apr 3-Oct
Dutch Harbor 1997 54.7 338 1-Jan 5-Dec
Fairbanks 1978 6.9 179 4-Apr 30-Sep
Juneau 2001 45.0 270 20-Feb 17-Nov
Ketchikan 1952 147.4 333 31-Jan 29-Dec
Kotzebue 1977 6.1 153 4-May 4-Oct
Nome 1997 10.4 167 21-Apr 5-Oct
Tok 1985 6.7 172 17-Apr 6-Oct
Valdez 1976 46.3 243 27-Mar 25-Nov


Appendix B

Calculation of Current Storage Condition

The current storage on any given day was calculated according to the following equation:

S (k ) = S (k − 1) + P(k − 1) ∗ SA − U (k − 1) , where

S(k) = Current day stored water volume
S(k-1) = Previous day stored water volume
P(k-1) = Previous day precipitation
SA = Rainwater catchment surface area
U(k-1) = Previous day water comsumption.

The equation quantified the available water at the start of any given day as a function of
the existing stored volume, the tank recharge, and water consumption. Two physical
constraints imposed on the modeled scenarios dictated that the total storage term could
neither be negative nor could it exceed a specified tank volume.

Please direct further inquiries regarding the modeling methods or results to the authors of
this study. They can be located at the University of Alaska Fairbanks Institute of
Northern Engineering, http://www.alaska.edu/uaf/cem/ine/.


Appendix C

Graphs of Optimal Rainwater Storage Volume for Twelve
Alaskan Locations


Alaska; Optimal Storage Volumes For Rainwater Catchment Systems In Alaska

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