THORNTHWAITE’S SCHEME

Thornthwaite’s climate classification scheme, developed by climatologist C.W. Thornthwaite in 1948, is a method used to classify climates based on the water balance approach. Unlike other climate classification systems that focus on temperature and precipitation alone, Thornthwaite’s scheme emphasizes the role of moisture availability in relation to evapotranspiration (potential water loss from the soil and plant surfaces). This classification system is particularly useful for assessing agricultural potential, water resources management, and understanding ecosystem dynamics.

Components of Thornthwaite’s Scheme

Thornthwaite’s classification is based on two main factors:

1. Calculation of Potential Evapotranspiration (PET):
   • Thornthwaite developed a formula to estimate potential evapotranspiration based on temperature and day length. This calculation considers how much water would evaporate from the soil and transpire from plants under ideal conditions.
2. Effective Moisture (EM):
   • This is the difference between precipitation (P) and potential evapotranspiration (PET). It represents the water surplus or deficit in a particular climate zone over a year. Effective moisture index (EM) values determine the climate classification.

Classification of Climates

Thornthwaite classified climates into five major types based on the effective moisture index (EM):

1. Humid Climates:
   • Type A: No moisture deficit throughout the year (EM > 0).
     • Example: Regions with abundant and evenly distributed rainfall such as tropical rainforests.
   • Type B: Moisture surplus in the summer, potential deficit in the winter (EM > 0).
     • Example: Eastern United States, parts of Europe with warm summers and moderate rainfall.
2. Subhumid Climates:
   • Type C: Small moisture deficit in some months (EM slightly negative).
     • Example: Mediterranean climate regions with hot, dry summers and mild, wet winters.
3. Semiarid Climates:
   • Type D: Moderate to large moisture deficit for more than half of the year (EM significantly negative).
     • Example: Steppe regions of Central Asia and North America with sparse vegetation and erratic rainfall.
4. Arid Climates:
   • Type E: Severe moisture deficit year-round (EM highly negative).
     • Example: Sahara Desert in Africa, where precipitation is extremely low and evaporation rates are high.
5. Polar Climates:
   • Type H: Conditions with very low PET due to cold temperatures.
     • Example: Arctic regions where temperatures remain below freezing for much of the year, resulting in limited evapotranspiration.

Example Application: Mediterranean Climate

Let’s apply Thornthwaite’s scheme to understand the Mediterranean climate:

• Characteristics: Mediterranean climates typically have hot, dry summers and mild, wet winters. The precipitation pattern matches Thornthwaite’s Type C classification, with a potential deficit in the summer due to high evapotranspiration rates.
• Example Region: Southern California, USA
  • PET Calculation: Thornthwaite’s formula would estimate the potential evapotranspiration based on temperature and day length during the summer months, reflecting the high water loss potential.
  • Effective Moisture (EM): Despite having wet winters, the region experiences a moisture deficit in summer months due to low precipitation and high evapotranspiration rates.

Comparison with Other Classification Systems

Thornthwaite’s scheme differs from the Köppen Climate Classification system by focusing more on the hydrological aspects of climate, particularly the balance between precipitation and potential evapotranspiration. Köppen’s system, on the other hand, emphasizes temperature and vegetation types to categorize climates.

Conclusion

Thornthwaite’s climate classification scheme provides a nuanced understanding of climates based on water balance dynamics, making it valuable for agricultural planning, water management strategies, and ecological studies. By considering both potential evapotranspiration and precipitation patterns, Thornthwaite’s scheme helps to assess the suitability of different regions for various agricultural practices and provides insights into how climate influences ecosystem functioning and human activities.