Download Streamflow and Runoff ( file)

P R O J E CT G U T S
W AT E R R E S O U R CE S
Water Runoff Models
in StarLogo TNG
Introduction
Streamflow in the eastern Sierra
Nevada. Markleeville, CA. May
2000. Source:
http://pubs.usgs.gov/fs/2005/3018/
Runoff from snowmelt is a major component of the
water cycle. In New Mexico, much of the springtime
runoff and streamflow in rivers is attributable to
melting snow and ice. In the mountains, snowfields
act as reservoirs for the water supply. Water is stored
as snow during the cool season and melts during the
warm season. Melted snow forms the water in rivers.
As much as 75% of the water supply in western states
is derived from snowmelt. Some rivers go dry when
the supply of water coming from melting snow has
been exhausted. The lack of water stored as
snowpack in the winter can affect the availability of
water for the rest of the year. This can also have an
effect on the amount of water in reservoirs located
downstream, which in turn can affect water
availability for agricultural irrigation and water
supplies for cities and towns.
Snowmelt is affected by climate change. Runoff from snowmelt varies with
season and also year to year. In recent decades, annual streamflow has come
progressively earlier. Trends towards diminished snowpack and earlier snowmelt
may be related to global warming. These trends correlate with a regional trend
towards warmer winters and springs in the same periods. Changes towards
earlier snowmelt and streamflow threaten water resources in our state.
In 2025 A Water Prophecy, the Middle Rio Grande Water Assembly describes a
future scenario
“The first problem -- once climate refused to live up to our expectations —
was that all our eggs were in the ‘holding back snowmelt’ basket. We had a
string of reservoirs designed to store and release spring runoff for use in the
dry months. And suddenly there was damn little snow. Nevertheless, we
were obligated by countless treaties and documents to send water down
the river, even if it meant turning off the tap on everything in the central
valley, lining parts of the river, and pumping groundwater solely to feed the
river.”
PAGE 2
INTRODUCTION
They based their story on the following projections. Streamflows into the middle
Rio Grande from snow-fed headwaters will decrease by 2025, due to diminshed
snowpack and increased evapotranspiration. Different scientific models project
decreases in average annual runoff, relative to late 20th Century average flows,
ranging from about 3% to more than 10%. While the range of these projections is
large (3% - 10%), all of the projections trend downward. While total annual
economic losses are estimated in the vicinity of $300 million, under severe
climate changes, where runoff is reduced by nearly 30%, both economic and
non-economic losses are likely to be significantly higher.
Sources:
The Water Cycle: Snowmelt Runoff
http://ga.water.usgs.gov/edu/watercyclesnowmelt.html
Changes in Streamflow Timing in the Western United States in Recent Decades
http://pubs.usgs.gov/fs/2005/3018/
2025 … A Water Prophecy? “The Story”
downloaded from http://www.waterassembly.org/11assembly.htm
Annual Water Report of the Sangre de Cristo Water Division
Climate Change and the Mountain Snowpack Reservoir, Caiti Steele, ppt presented at STI 2011.
Climate Change and Its Implications for New Mexico’s Water Resources and Economic Opportunities.
NMSU Technical Report 45 Brian H. Hurd and Julie Coonrod.
INTRODUCTION
PAGE 3
The Runoff Models:
The runoff model is an “idea” model that simulates a part of the hydrological
cycle in which snowmelt has become streamflow and is captured by a gravity
fed ditch or acequia for agricultural or human use.
The question we can address in this model is whether early fast runoff captures
less water than more moderate later runoff.
We start with a very simple model of a river of fixed width and water agents
representing units of water. In Spaceland we use an overhead view in 2
dimensions with the stream running top to bottom. Water agents are created
and populate the stream. The water agents move downstream with some
randomness in their movement.
Version 1: Water flowing downstream and capture into acequia that leads to a
reservoir.
Initially, the environment
consists of a streambed
indicated in cyan and a
gravity fed ditch or pipe in
yellow. The resevoir at the
end of the ditch is colored
red.
We’ve set the width of the
streambed arbitrarily at
this point.
In the Setup procedure, we create water agents,
color them blue, and call the Fill procedure to
populate the stream with water agents. Then we
initialize two global variables: water_runoff and
water_captured. Water_runoff counts the number
of water agents that have reached the bottom of
the stream and Water_captured counts the
number of water agents that have been captured
at the resevoir at the end of the acequia.
PAGE 4
INTRODUCTION
In the Fill procedure, water agents are given an xcoordinate and a y-coordinate. The Fill
procedure tests to see if the water agent has
landed in the streambed. If it has not, then the Fill
procedure is called again recursively.
In the forever loop three procedures are called on
water agents. The “gravity” procedure
implements water agents’ movement
downstream; the “recycle” procedure recirculates water agents that reach the bottom of
the stream; and the “drain” procedure captures
and counts the water agents that hit the reservoir.
In the gravity procedure we
first check to see if the ycoordinate of an agent is 50.
If so, it is at the top of the
stream and should be visible.
The water agent is given a
random downward heading
and we test to see if the
patch ahead is brown,
indicating the shoreline. If so,
we simply try another
direction. If not, we test to
see if the patch ahead is
cyan, indicating streambed.
If the streambed is ahead of
the agent, the agent takes
two steps forward and sets its
direction again. If there is a
water agent there already,
we move the agent back
one step.
INTRODUCTION
PAGE 5
Finally, we test to see if the water agent is at the ditch or pipe. If so, we set the
agent to move horizontally along the pipe until it reaches the reservoir.
In the recycle procedure, we test to see if
the water agent has reached the bottom of
the stream. If so we hide it, increment the
water_runoff variable and set it to the top of
the screen.
The drain procedure simply checks if the
agent is at the reservoir as indicated by a
red patch color. If so the water_captured
variable gets incremented and the agent is
recycled.
Assumptions:
In this simplest version, there are many assumptions made.
- There is a steady supply of water with all water being recycled.
- The width of the streambed is set rather than being generated by the
water agents.
- The streambed is shallow and flat rather than having any depth.
- A water agent represents a unit of water, what unit?
- Slower flow is created by changing the agent step size.
- Temperature fluctuations and icepack levels are outside the scope of this
first model.
How to use this model:
Students can create different versions of this model (fast vs. slow) with different
streambed widths then run experiments to collect data on the amount of water
captured in each case. Is there a relationship between the width, flow speed,
and water captured.
PAGE 6
INTRODUCTION
Next steps:
Students may want to alter this base model to test other hypotheses. For
example they could add more acequias up and down stream from the current
one. They could branch one acequia into multiple receiving acequias or
agricultural fields.
Extensions:
To investigate whether early fast runoff captures less water than more moderate
later runoff, we will have to make many changes to the model.
-
implement seasonal flow.
Implement variable stream width depending on flow rate.
Image excerpted from Climate Change and the Mountain Snowpack Reservoir, Caiti Steele, ppt presented at STI
2011.