A vineyards degree of slope can be very important for several reasons: air flow through the canopy, soil drainage, level of water movement and possible erosion, and ease of working in amongst the vines, managing with equipment and then ease of harvesting. There is no perfect slope as it will depend on what is the primary limiting factor of concern.
The further north or south - the greater the latitude from the equator the steeper the slope needs to be for ideal sun exposure and heating of the vines. The slope or inclination is measured as a percentage of elevation change over a horizontal distance. Therefore for example: a perfectly flat vineyard would have a slope of 0% and vertical cliffs would have a slope of 100%.

 

A slight to moderate slope of 5% to 10% is desirable for vineyard sites as it encourages the drainage of denser cold air from the vineyard. Cold air is denser than warm air and will drain downhill, if there are no barriers to air movement such as trees or berms. Vineyards sloping greater than 15% become much more difficult to manage, as it is hazardous to operate machinery on steep, fragile slopes and these sites can erode more easily.
Slope ranges include:
Flat to 2.5%: Easy to manage and harvest, little soil erosion, may be prone to cold air inversions in places with frost problems. 2.5 to 5%: Allows for adequate air drainage; may have some small erosion concerns. 5 to 7.5%: Allows for good air drainage; increasing erosion concerns; concerns for row orientation and equipment on slope. 7.5 to 10%: Allows for excellent air drainage; erosion and nutrient loss can be of concern; issues for row orientation and limit for equipment usage. 10 to 15%: Allows for excellent air drainage but with some increasing erosion and nutrient loss concerns; likely unsafe for equipment usage without some form of terracing or self-leveling equipment, diversion ditches to control run off, and rows oriented perpendicular to slope. A slope greater than 15%: Vine management becomes more difficult; plus erosion and equipment rollover can be of concern.
Hillside slopes, especially south-west facing slopes in the northern hemisphere, have always been the preferred location for growing quality wine grapes. It has been proposed that this was originally done out of necessity - that these lower fertile slopes were planted in grapes because other agricultural crops failed to ideally grow. These rocky, lower fertile soils produced small vines with less vigour than vines grown on deep, alluvial soils. Smaller vines resulted in smaller clusters and crop-weight, and the wines showed unique concentration and intensity. Hillside slopes are generally well-drained and have less frost issues in spring, as long as the cold air has somewhere to flow down and away too.
Though with these benefits, there are difficulties to consider in growing vines on slopes. Tractors and other equipment can find it difficult to work and stay upright and stable. With steep, terraced slopes you have limited choices regarding row direction and spacing. The slope incline, exposure, geology and topography will dictate where you can fit plant the vines.
On flat vineyards; activities like spraying, pruning, netting and harvesting a large area of vines is much easier. It is agreed by those who in the know, that placing a vineyard on a steep slope will dramatically increase the effort required to maintain the vineyard, and will significantly increase the investment needed to establish it. Terracing might be necessary, which can add a great deal of additional costs per hectare to the vineyard establishment.
Quality fruit can be grown on flat vineyards; the vines tend to be a little more vigorous and can require more pruning to keep the vines from getting too wild and woolly, which can take away the vines focus on the grapes.
Gentle slopes are preferred over flatter terrain, as vines growing on a slope can receive a greater intensity of the sun's rays, with sunshine falling on an angle perpendicular to the hillside. On flatter sites, the intensity of the sunlight is diluted as it spreads out across a wider surface area. Small slopes that are elevated above surrounding ground are ideal sites for vines, because these small elevations are less prone to frost. Additionally, a slope affords better drainage, reducing the possibility that the vine might sit in overly moist soil. In cooler regions of the northern hemisphere, south-west facing slopes receive more hours of sunlight and are preferred; in warmer climes, north-facing slopes are preferred. In the southern hemisphere, these orientations are reversed.
An example used to good use to show the influence of flat vs sloping vineyards is the wine region of Burgundy. Most of the vineyards of Burgundy are planted on slopes and they can be relatively steep, reaching a 35% grade near the regions upper heights. The soil on the highest slopes has a thin topsoil and receives the least amount of rain fall during the growing season. The middle to higher part of the slopes receive the most exposure to sunlight and have the best drainage so they are designated as 'Grand Cru' vineyards. The Premier Cru are below Grand Cru vineyards on the slopes while the Village wines are produced from the flat territory which has the least amount of sun exposure and poorest drainage.
A wine region which has worked well with extremely steep slopes is the Douro Valley of Portugal. In terms of vineyard layout, planting in the Douro poses a multitude of problems associated with very steep and rocky terrain which is more suited to breeding goats than growing grapes. Solving these issues is an ongoing process - they are shaped largely by the availability of funds, labour and technology. If possible having vines on both slopes and vineyards which are relatively flat, gives options to the winemaker on varietal diversity, ripeness and grape flavours.