Water in the Ground

सिकाइ उद्देश्यहरू

• Describe infiltration, evaporation and transpiration as components of the water cycle.
• Record soil moisture at regular intervals and construct a time-series graph.
• Calculate the rate of moisture change and interpret what drives it.
• Explain the link between soil water storage, irrigation and Nepal's terraced landscape.

पृष्ठभूमि

When rain falls on soil, water can take three paths: it can run off the surface (runoff), be absorbed into the soil (infiltration), or be intercepted by plant leaves and evaporate directly. Infiltrated water is stored in the tiny spaces (pores) between soil particles. From there it can be absorbed by plant roots (transpiration), evaporate from the soil surface, or drain downward to groundwater (percolation). Together, evaporation and transpiration are called evapotranspiration — the combined return of water from land to atmosphere.

Soil texture determines how much water it can hold. Clay soils have tiny pores that hold water tightly but drain slowly. Sandy soils drain quickly but hold little water. Loam — a mixture of sand, silt and clay — is ideal for most crops as it holds adequate moisture while still draining freely. Much of Nepal's productive hill soil is loam enriched by centuries of organic matter from composted crop waste and animal manure.

The rate at which soil loses moisture tells us about energy in the environment. On a hot, sunny day with low humidity, evapotranspiration is high and soil dries quickly. On a cool, cloudy, humid day — or after a monsoon rain — the soil stays wet for much longer. This is why monsoon-era soils remain productive for weeks after the rains stop: the soil acts as a reservoir.

विधि

1. Record soil moisture now: ___ %. Record the time: ___
2. Calculate: at what soil moisture level does the soil label change from "Moist" to "Dry" on the dashboard? (Read the threshold from the display.)
3. Record moisture readings every 10 minutes for 30 minutes (3 readings total). Record temperature each time too.
4. Plot your three moisture readings on a line graph (time on x-axis, moisture % on y-axis).
5. Calculate the rate of moisture change: (final reading − first reading) ÷ (time in hours) = ___ %/hour.
6. Predict: if this rate continued, when would the soil reach 20% moisture? Show your calculation.
7. Discuss: what factors in the room or outside are causing the moisture to change? How would the rate differ on a rainy day versus a hot dry day?

छलफल प्रश्नहरू

1. After heavy monsoon rain, a field might reach 90% soil moisture. Using your measured rate of change, estimate how many days it would take to dry to 40%. What assumptions does this estimate require?
2. Why do terraced rice paddies need to maintain very high soil moisture (near saturation) during the growing season?
3. What would happen to Nepal's hillside farms if the monsoon arrived two months late?

कार्यपत्र

शब्दावली

Infiltration

The movement of water from the soil surface downward into the soil profile.

Evapotranspiration

The combined loss of water from soil evaporation and plant transpiration to the atmosphere.

Field Capacity

The soil moisture level after excess water has drained away — typically the upper limit of useful moisture for plants.

Wilting Point

The minimum soil moisture below which plants can no longer extract water and begin to wilt permanently.

विस्तार

Research the Penman-Monteith equation — the standard method used by agronomists worldwide to estimate evapotranspiration from temperature, humidity and wind speed. Which of these variables can our sensor hub measure? What additional sensors would be needed for a complete FAO-standard calculation?