Understanding evapotranspiration and its role in soils, plants, and the atmosphere

What happens to water in the soil? A plain‑spoken guide to evapotranspiration and Maryland soils

If you’ve ever stood in a field after a summer rain and wondered where all that moisture went, you’re not alone. Water in the ground doesn’t just sit there with a clock ticking on it. It moves—up, out, and away—in a delicate dance that scientists call evapotranspiration. For anyone juggling soil health, nutrient balance, and crop yields in Maryland, understanding this term isn’t just academic. It’s practical, it’s visible, and it helps explain why some fields stay damp while others dry out, even with similar weather.

What does evapotranspiration really mean?

Let me explain in two quick beats. Evapotranspiration, often shortened to ET, describes the net loss of water from soils to the atmosphere. It’s a combination of two processes:

• Evaporation: water in the soil (or on water surfaces) turns into vapor and escapes into the air. Think of a damp surface after a rain drying out in the sun.

• Transpiration: water absorbed by plant roots makes its way up through the plant and exits as water vapor from the leaves. The tiny openings on leaves—stomata—are doing a lot of quiet work there.

Put together, ET is the main way water leaves the ground and returns to the atmosphere. In Maryland’s climate, ET rates swing with the season, the crops you’re growing, wind, and how wet or dry the soil is. The higher the ET, the more water leaves the soil, which can affect soil moisture and the movement of nutrients.

A quick tour of the water cycle terms you’ll see around Maryland fields

To keep this grounded, here are a few related terms and how they differ from ET:

• Infiltration: water soaking into the soil. It’s the reset button for soil moisture—water entering the soil profile from the surface, which can later contribute to ET as it moves through the system.

• Condensation: water vapor cooling and turning back into liquid, often forming clouds. You’ll hear about the water cycle in a broader sense, but condensation isn’t the primary driver of water loss from soils.

• Precipitation: rain, snow, sleet—water input from the sky. Precipitation fills soils and can replenish moisture that ET later removes.

The key distinction is: ET is a loss term for soil water going to the atmosphere, while infiltration and precipitation are inputs, and condensation is a phase change you’ll hear about in meteorology more than in the soil lab.

Why ET matters for Maryland nutrient management

Here’s the practical angle: water doesn’t just vanish. It carries dissolved nutrients with it or can push nutrients deeper into the soil or toward surface runoff. ET is the engine that controls how fast soil dries out, which in turn influences nutrient availability, microbial activity, and how fertilizers move through the root zone.

• Soil moisture affects nutrient transport: when soil is wet, nutrients can move with water (a process called leaching). When soils dry out, roots may grow deeper in search of moisture and nutrients, changing nutrient uptake patterns.

• Plant water needs and nutrient demands align with ET: crops drink more water when ET is high, but their ability to take up nutrients can either keep pace or lag behind, depending on soil texture, organic matter, and management.

• Local climate and weather feedback: Maryland’s hot, humid summers and variable rainfall create ET pulses that matter for both irrigation planning and nutrient budgeting. ET helps explain why a field that looks fine after a rain can still suffer moisture stress a week later.

Think of ET as a governor for how much water stays in the root zone. Since nutrients ride along with the water in the soil, ET indirectly guides how and when nutrients are available to crops.

How ET works in the real world: evaporation and plant transpiration in action

Let’s break down the two components with a farmer’s eye:

• Evaporation from soil and water surfaces: this is the quick, surface‑level loss. If a field surface is bare soil, high temperatures and wind can push water into the air pretty fast. Mulches, crusts, and plant cover can slow this down by shading the ground and reducing evaporation.

• Transpiration through plants: roots take up water that travels up the plant and leaves exhale water vapor. This is not just thirsty crops; even grasses and cover crops can contribute a meaningful slice of ET. The rate depends on plant species, leaf area, stomatal conductance, and how stressed the plant is by moisture or heat.

In practice, a field with good ground cover tends to lose less water through evaporation, at least from the soil surface, because the canopy and mulch act as a shield. But a vigorous, well‑watered crop can still contribute a hefty share of ET through transpiration. Both streams end up in the atmosphere, and both shape how much water remains available to soils and crops.

A Maryland lens: what ET means for soil moisture and nutrient flow

Maryland’s soils are diverse—from clayey, holding lots of water to sandy profiles that drain faster. ET interacts with that texture in telling ways:

• In heavy soils, high ET may still leave a nice moisture reserve, but evaporation from the surface can keep drying the top layer. Plants then pull from deeper layers, potentially moving nutrients into new zones.

• In sandy soils, ET can quickly remove surface moisture, which can stress crops sooner during dry spells; nutrient movement with percolating water becomes more pronounced—sometimes beneficial, sometimes risky if you’re trying to limit leaching.

Ground cover—think cover crops, crop residues, or living mulch—acts like a thermostat: it can dampen ET peaks by shading soil and reducing evaporation, while still supporting photosynthesis and nutrient capture when weather cooperates. That’s why many Maryland growers integrate cover crops or residues into their nutrient management plans: they help balance soil moisture, reduce erosion, and moderate nutrient losses.

Common misunderstandings (and how to clear them up)

• ET and infiltration are not the same thing. Infiltration is about water entering the soil; ET is water leaving the soil. They’re part of the same system, but they move in opposite directions.

• Precipitation is input, not output. It’s what adds water to the soil, not what takes water away.

• Condensation isn’t a major driver of soil water loss. It’s more about clouds and weather patterns. ET is the main sink for soil moisture in most field scenarios.

• ET isn’t a fixed number. It changes with the weather, crop type, soil health, and management. That variability is why irrigation schedules and nutrient strategies must stay flexible.

Practical takeaways for land managers and students

• Monitor soil moisture and weather: knowing how wet the soil is and what the forecast looks like helps you estimate ET indirectly. Tools like soil probes, weather stations, and simple field observations go a long way.

• Match crop choices to soil and climate: deeper rooted crops may access moisture and nutrients differently as ET shifts through the season. Choosing crops with appropriate water and nutrient needs keeps you in a better balance.

• Use cover crops and residue wisely: ground cover lowers evaporative losses and can anchor nutrients, reducing leaching risks during high ET periods.

• Don’t forget the nutrients: as ET moves water, it shifts where nutrients end up. Nitrogen, phosphorus, and potassium behave differently in soils; aligning fertilizer timing with ET patterns helps crops use nutrients efficiently and minimizes losses.

• Plan irrigation with ET in mind: optimal irrigation timing coincides with soil moisture deficits and weather patterns that push ET upward. The goal isn’t just to replace water lost to ET but to keep root zone moisture at levels that sustain healthy growth and nutrient uptake.

A handy mental model you can carry into the field

Picture ET as a two‑part faucet that pours water into the atmosphere: one spigot for evaporation off the soil surface, the other for plant transpiration from leaves. The amount of water you actually have in the root zone depends on how much comes in (precipitation and infiltration), how much goes out (ET), and how well the soil can store it (soil texture, structure, and organic matter). When you tune management to this flow—cover crops, residue management, smart irrigation—you’re not just keeping crops happy; you’re steering nutrient behavior in the soil.

A final thought: learning this isn’t just for “experts” with fancy equipment

Yes, ET is a scientific term, but its implications are surprisingly down‑to‑earth. It helps explain why your field looks damp after a rain but dries rapidly on a sunny afternoon. It informs decisions about fertilizer timing, irrigation, and soil stewardship. And in Maryland, where weather can flip quickly and soils vary from one county to the next, a practical grasp of ET translates into better yields, steadier soil health, and a more resilient farming system.

If you’re curious to see ET in action, you can explore simple demonstrations—like comparing soil moisture beneath bare soil, living mulch, and a cover crop corridor after a warm, dry spell. You’ll notice that moisture in the root zone behaves differently under each scenario, and you’ll start to see the water cycle not as a distant meteorology concept but as something you can observe, measure, and influence on your own land.

Resources to keep exploring

• Local extension services and soil scientists can help tailor ET considerations to your Maryland site conditions.

• Weather and soil moisture observations: state environmental agencies and university extensions often share practical data you can apply to field decisions.

• Nutrient management guidelines from state agencies and agricultural universities emphasize the link between water movement and nutrient availability, helping you craft strategies that support both crop health and environmental stewardship.

In the end, evapotranspiration isn’t some abstract science topic tucked away in a textbook. It’s the everyday reality of fields and farms, a quiet chorus of water leaving the ground and lifting into the air. By paying attention to ET and its siblings in the water cycle, you gain a clearer lens on soil moisture, nutrient dynamics, and the smart choices that keep Maryland soils productive and resilient. And that’s a perspective worth carrying from sowing to harvest—and beyond.