Lecture 4: Environmental Systems

Question 1

What is a system?

Answer 1

A collection of things that have some sort of relationship with each other and are contained within a defined boundary.

Question 2

What are the components of a system?

Answer 2

Elements, linkages, and a boundary.

Question 3

What are elements in a system?

Answer 3

entities or state variables. ex: atmosphere or ocean in the hydrological system.

Question 4

What are linkages in a system?

Answer 4

Couplings: cause effect/relationships or exchanges of material/energy. ex: precipitation or evaporation in the hydrological system.

Question 5

What is a boundary of a system?

Answer 5

Where the system ends. ex: a lake system in the hydrological system.

Question 6

What is an open vs closed system?

Answer 6

Open system: energy and mass can be exchanged (flux/flow)
Closed system: only energy can be exchanged (no mass exchanged)

Question 7

What is steady vs transient state?

Answer 7

Steady state: input = output
transient: input =/ output (there is some accumulation or loss of energy/matter).

Question 8

What is residence time?

Answer 8

The average length of time a substance remain in a reservoir at steady state: residence volume/net flow rate

Question 9

Ex: a university has 14,000 students in undergrad. If no fail/drop out and the average student takes 4 years to graduate, how many students graduate each year?

Answer 9

14,000 students / 4 years = 3500 students per year

Question 10

Ex: if there are 100 students enrolled in a graduate program,and each year, 20 graduate, 5 drop out, and 25 new students enter, what is the average residence time of a student in the program?

Answer 10

100 students/(20+5) students leave/year = 4 years years

Question 11

Ex: a population of cows are at steady state. If the birthrate is 7 calves per year and the average cow lives 6 years, how large is the heard?

Answer 11

7 calves/year x 6 years = 42 cows in the heard

Question 11

Ex: a pollutant enters the lake at 0.16 tonnes per day. The lake volume is 4x10^7 m3 and the average water flow is 8x10^4 m3. What is the residence time of the pollutant?

Answer 11

Assume water residence time = pollutant residence time (since its completely dissolved).
4x10^7 m3 / 8x10^4 m3/day = 500 days

Question 12

Ex: a pollutant enters the lake at 0.16 tonnes per day. The lake volume is 4x10^7 m3 and the average water flow is 8x10^4 m3. What is the total stock of pollutant at steady state?

Answer 12

0.16 tonnes/day x 500 days = 80 tonnes