Problem 9 - Campbell Biology

Question

(a) Draw a simplified global water cycle showing ocean, land, atmosphere, and runoff from the land to the ocean. Label your drawing with these annual water fluxes: \(\cdot \) ocean evaporation, \(425 \mathrm{~km}\) ocean evaporation that returns to the ocean as \(\cdot \) precipitation, \(385 \mathrm{~km}^{3}\) \(\cdot \) ocean evaporation that falls as precipitation on land, \(40 \mathrm{~km}^{3}\) \(\cdot \) evapotranspiration from plants and soil that falls as precipitation on land, \(70 \mathrm{~km}^{3}\) \(\cdot \) runoff to the oceans, \(40 \mathrm{~km}^{3}\) (b) What is the ratio of ocean evaporation that falls as precipitation on land compared with runoff from land to the oceans? (c) How would this ratio change during an ice age, and why?

Step-by-Step Solution

Answer

The ratio is 1. During an ice age, the ratio would increase because runoff would decrease.

Step 1: Understand the Water Cycle Components

Identify the main components of the global water cycle: ocean, land, atmosphere, and runoff. Understand the fluxes provided: ocean evaporation, ocean precipitation, land precipitation from ocean evaporation, and evapotranspiration leading to land precipitation.

Step 2: Create the Diagram

Draw the main components: the ocean, land, and atmosphere. Use arrows to represent fluxes: 425 km³ for ocean evaporation, 385 km³ for ocean evaporation returning as ocean precipitation, 40 km³ for ocean evaporation falling as precipitation on land, 70 km³ for evapotranspiration, and 40 km³ for runoff to the ocean.

Step 3: Label the Diagram

Label each arrow in the diagram with the corresponding water flux values: 425 km³ for ocean evaporation, 385 km³ for precipitation back into the ocean, 40 km³ for ocean evaporation falling on land, 70 km³ for land precipitation from evapotranspiration, and 40 km³ for runoff to the ocean.

Step 4: Calculate the Ratio

Find the ratio of ocean evaporation that falls as precipitation on land (40 km³) to the runoff from land to oceans (40 km³). The ratio is \(\frac{40 \text{ km}³}{40 \text{ km}³} = 1\).

Step 5: Consider Ice Age Conditions

During an ice age, more water is trapped in ice on land, reducing runoff. Consequently, the ocean evaporation falling as precipitation on land would be higher relative to reduced runoff, increasing the ratio. Thus, the ratio would be greater than 1.

Key Concepts

ocean evaporation

Ocean evaporation is a crucial component of the global water cycle. When the sun heats the surface of the ocean, it causes water to evaporate and turn into water vapor. This process removes water from the ocean. In the context of the exercise, we see that 425 km³ of water evaporates from the ocean annually. This water vapor then enters the atmosphere, playing a key role in forming clouds and eventually precipitating back to the Earth's surface. It's important to grasp that ocean evaporation is the primary source of atmospheric moisture, driving the cycle forward both over the ocean and the land.

precipitation

Precipitation refers to any form of water that falls from the atmosphere back to the Earth's surface, including rain, snow, sleet, and hail. The exercise highlights two types of precipitation: 385 km³ of ocean evaporation returns as precipitation directly to the ocean, while 40 km³ falls on land. Additionally, evapotranspiration contributes 70 km³ to land precipitation. Precipitation over land replenishes freshwater sources like rivers, lakes, and groundwater. The interplay of atmospheric moisture returning to the Earth in different regions underscores the balance maintained in the water cycle.

evapotranspiration

Evapotranspiration is the combination of two processes: evaporation from the soil and water bodies, and transpiration from plants. Plants take up water from the soil and release it as water vapor through tiny pores called stomata during photosynthesis. This dual mechanism contributes significantly to the moisture in the atmosphere. In the exercise, we see that evapotranspiration accounts for 70 km³ of water that eventually precipitates on land. Understanding this concept is essential for appreciating how plants and soil interact with the atmosphere, keeping the local and global water cycles in motion.

runoff

Runoff is the flow of water, typically from precipitation, that travels over the land and drains into bodies of water like rivers, lakes, and ultimately oceans. This process transfers water from the land back to the ocean, completing the water cycle. In the scenario provided, 40 km³ of water becomes runoff making its way back to the ocean. Runoff is critical for maintaining river flows, replenishing freshwater supplies, and affecting the levels of water bodies. During special conditions like an ice age, the amount of runoff can be significantly altered, affecting the overall equilibrium of the global water cycle.